Compression of 3D Triangular Meshes with Progressive Precision In this paper we introduce a novel approach for progressive transmission of three-dimensional (3D) triangular meshes. This algorithm is based on a new reversing approach of the irregular mesh subdivision that enables a wavelet representation of any mesh geometry. In this paper, we show how to achieve progressive compression of 3D models by transmitting more and more wavelet coefficients computed from the original mesh vertices coordinates. The connectivity of the reconstructed mesh remains the same as the original one, but its geometry is progressively refined by means of bitplane encoding. This approach processes directly floating point coordinates which is the most common representation for 3D meshes, and does not need quantization, which is a lossy transformation. Experimental results are given and demonstrate the efficiency of our encoding scheme versus other approaches. r Digital models of three-dimensional (3D) shapes are essential in numerous application domains, including CAD, entertainment, medicine, geosciences, and architecture. In most of the cases, these models are represented as 3D triangular meshes with the native support provided by graphics processors. However, the use of large and complex models can become an issue when the end-user faces storage or bandwidth limitation. Data compression offers a solution for these problems by providing two different approaches: The model can be compressed either in a single-resolution bitstream, or in a progressive bitstream. Single-resolution approaches [1-3] often give the best compression rate, but progressive schemes [4] [5] [6] [7] [8] [9] [10] [11] offer more flexibility (for a given model several resolution levels are constructed so that progressive transmission and reconstruction are possible). Also, lower resolution levels can be rendered faster which may be useful for large models.. In this paper, we propose a new approach for progressive transmission of 3D models based on the decomposition of the mesh geometry on a geometrical wavelet basis. The main contribution proposed in this paper is a progressive transmission of the mesh geometry from the lowest to the highest-resolution level while keeping its connectivity unchanged. During reconstruction of the compressed model, the geometric refinement is obtained by progressively transmitting details (wavelet coefficients), and a good reconstruction is performed with only a small proportion of the wavelet coefficients set. In sharp contrast with other work, our approach can directly compress the vertices coordinates, without any quantization step. The paper is organized as follows: in Section 2 we briefly discuss the previous works related to 3D mesh compression. Section 3 explains why quantizing the vertices coordinates provides a tradeoff between efficiency and accuracy. Section 4 describes the main parts of our algorithm. Experimental results are shown, and a comparison is made versus other approaches in Section 5. Finally, Section 6 concludes the paper and some perspectives are given. In this paper, we have proposed a novel approach for progressive compression of 3D triangle meshes. Our approach can be applied directly to irregular meshes without applying any remeshing or quantization step, which makes it an asymptotically lossless coder for floating-point vertices coordinates. The algorithm applies a wavelet transform on vertices coordinates, which are further encoded with a zerotree coder. Experimental results show that our algorithm achieves good performances with a low computational complexity. Our approach is based on progressive transmission of wavelet coefficients with zerotree encoding. An irregular subdivision scheme reconstructs the original mesh connectivity graph. Further work could address the construction of an improved Butterfly interpolating scheme adapted to the irregular subdivision scheme, in order to improve the efficiency of the proposed approach for highly irregularly sampled models or models with sharp features. We have tested our algorithm on several reference models. Our implementation is fast. As an example, the compression of the Bunny mesh with 34 834 vertices takes less than 10 s on an Intel Pentium II laptop computer running at 400 MHz with 128 MB RAM. In Fig. 6 , we compared the rate-distortion efficiency of our approach when applying several different lifting schemes: only primal lifting, only dual lifting, or both primal and dual lifting. For the regularly sampled Venusbody (11 362 vertices, Fig. 6(a) ), using both primal and dual lifting significantly improves the efficiency of our approach. On the irregular Bunny (34 834 vertices, Fig. 6(b) ), dual lifting does not improve the quality of the results; moreover, at high bitrates, it penalizes the global performances of our approach. We believe that this issue is caused by the Butterfly lifted wavelets which were initially created for regular 1:4 subdivision. Adaptation of Butterfly lifted wavelets to irregular subdivision schemes should be considered, as explained in [26] . Fig. 2 compares our approach with results obtained by the improved wavemesh [27] using several quantization steps, on the Venusbody and the Horse (19 851 vertices). Note that for each model, only one curve is drawn for our approach, as quantization is not needed. On the Venusbody, the proposed approach outperforms all results obtained by wavemesh. Good performances result from the fact that this model is very smooth and its connectivity is highly regular. On the other hand, the Horse model has very irregular connectivity. As a consequence, encoding the connectivity of the mesh costs much more than for the Venusbody. That is why the rate-distortion curve begins above 4 bits/vertex (this is the encoding cost for the connectivity). For bitrates above 5 bits/vertex, our approach achieves a good rate/ distorsion tradeoff. As an example, applying Wavemesh with 14 bits/coordinate quantization gives the best results above 20 bits/vertex, but performs poorly for low bitrates. On the other hand, our approach achieves Wavemesh [11] , for the Venusbody model (11 362 vertices, 10 bits quantization) and for the Venus head (8268 vertices, 12 bits quantization). It can be seen that our approach performs similar to or better than spectral compression, for a much lower computational complexity. Note that the results given for our approach were obtained without quantizing the mesh, thus guaranteeing that we can reach higher precision than other works.. Figs. 8 and 10 show different reconstruction levels for the Venusbody and Venus head meshes. Clearly, the Venus head mesh is not smooth at low bitrates, due to its very irregular sampling. On the other hand, the Venusbody mesh is well reconstructed even at low bitrates. Finally, we compare our approach with the Progressive Geometry Coder (PGC) of Khodakovski et al. [9] in Fig. 9 . This figure shows the rate-distortion efficiency on the Venus model (50 002 vertices) for PGC, and two curves are given for our approach: the first one results from compressing the original Venus model, and the second one results from compressing the remeshed model used by PGC. The results clearly show that the remeshed model is significantly better compressed than the original one. As expected, remeshing is effective when one wants to keep only the geometry of 3D models. Single-resolution compression schemes for 3D meshes usually create a single bitstream, which can be split into two parts: the connectivity bitstream (which describes the mesh connectivity graph) and the geometry bitstream (the vertices coordinates) (see Fig. 1(a) ). Progressive transmission of meshes involves splitting both the bitstreams into several components. The connectivity bitstream usually contains a base mesh which is further refined by reading the successive bitstreams. The geometry bitstream is also decomposed into a base geometry and several geometrical refinements. Fig. 1 (b) depicts how these bitstreams are interleaved during transmission.. Progressive transmission for meshes, introduced by Hoppe [7] , is based on successive mesh simplification by means of edge contractions which offer optimal granularity and linear complexity, but non-linear storage and transmission cost. Several approaches derive from progressive meshes: Pajarola and Rossignac [10] implemented an improved version of progressive meshes, encoding the mesh connectivity reconstruction by edge split batches, with an average coding cost of 7.2 bits/vertex. Karni et al. [8] improved the edge contraction sequence and the geometry coding to enhance both the progressive transmission rate-distortion tradeoff and the rendering speed of the processed meshes. Cohen-Or et al. [5] proposed a progressive transmission based on successive vertex removal followed by deterministic retriangulation. Vertices are removed according to their valence and geometric properties. The deterministic retriangulation leads to an average connectivity compression of 6 bits/vertex, but the simplified mesh becomes less and less regular due to the removal of high valence vertices. Alliez and Desbrun [4] improved this technique by removing only vertices with valence below 7 and using a new retriangulation approach that maintains the mesh regularity along the simplification. The vertices can also be removed according to a geometric criterion to improve the quality of the approximations. This approach compresses the mesh connectivity to an average of 3.69 bits/vertex. Karni and Gotsman [12, 13] developed spectral geometry compression, where the geometry is projected on to an orthogonal vector space, constructed with the eigenvectors of the mesh connectivity Laplacian matrix. This scheme provides good mesh approximations, even with few transmitted coefficients. The main drawback of this scheme is the need to calculate eigenvectors of the mesh connectivity graph for which computational complexity is not linear to the number of triangles, but to its cube. This drawback was solved partially in [13] where precomputed eigenvectors are used, using a planar mapping of the mesh connectivity graph on a regular graph. In [11] Valette and Prost introduced the Wavemesh approach, which uses an irregular subdivision inversion scheme [14] . This scheme aims at processing an inverse 1-to-4 subdivision [15] on the input mesh whenever it is possible. The geometric approximations are performed by wavelet decomposition, which enhances the approximation quality of the lower-resolution levels.. In [9] , Khodakovsky et al. presents a pure geometry coder, where the input model is remeshed into a semiregular one. This approach is very efficient, as it removes the connectivity information of the input mesh when the user does not need to keep the original connectivity of the 3D mesh. Single-rate lossy compressions that resample the surface in a semi-regular pattern have also been proposed [16, 17] .. In [18] , Guskov et al. describe a new 3D representation. A normal mesh is a mesh in which geometry can be represented mostly by normal components and is then suitable for progressive compression. Again, this approach provides very good results, at the cost of a remeshing of the compressed model. This approach was recently improved in [19] .. Note that remeshing progressive approaches [9, 18] do not retain the original connectivity of the mesh, and the compressed bitstream mainly consists of geometrical refinements (except the base mesh connectivity) as shown in Fig. 1(c)

O O O B I O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O B O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Citation-based journal ranks: The use of fuzzy measures Keywords: Aggregation operators, Citation analysis, Choquet integral, ordinal classification.This article considers a real world application of the Choquet integral based classifier to the problem of ranking of scientific journals. We attempted to predict rankings given by panels of experts, by using available citation indices. We compare the Choquet integral fit by the least absolute deviation criterion as a classifier against other ordinal classification methods, before interpreting fuzzy measure weights and indices in this application. Our findings show that the journal rankings data set is difficult to model accurately due to inconsistencies and lack of monotonicity, but that the Choquet integral still performs well as a classifier. Assessment of the quality of research is a growing trend in many countries. The metrics for such assessment include external grants, PhD completions, patents, the international prestige of researchers, and, of course, research output. The latter is typically measured in terms of the quantity and quality of refereed publications (both journal and full conference papers), sometimes using citation analysis. The focus on quality of journal articles poses an important question of how quality should be measured. Peer review of a sample of key research papers for each researcher in an organization is probably the most accurate method (although it is also prone to misuse), but when the assessment exercise is done on the scale of the whole country, the cost of this approach is prohibitive. Thus assessment panels recur to more crude but less expensive methods.. Citation analysis is a crude metric to assess the quality of individual articles. The rationale is that highly cited papers have greater impact and thus have higher value. This rationale has been strongly criticized. For instance [19] studied a set of highly cited publications on clinical trials, and found that for about 32% of the items, the results were later contradicted. It is known that reviews and meta-analysis receive the largest number of citations, and that monographs are still the major reference source, although citations to monographs are not counted by the major citation data providers (Thomson ISI and Scopus).. The second approach is to judge the quality of publications based on some measure of the quality of journals they were published in. This approach is criticized from two angles. First, the quality of a journal does not necessarily imply the quality of a particular article. Second, the metrics used to determine the quality of journals are usually citation-based (in particular, based on its Impact Factor (IF)). It has become customary to include journals' IF and their ranking based on IF in CVs, tenure and funding applications, etc.. Citations also form the basis of personal indices such as Hirsch's h−index [16] and similar calculations which attempt to quantify individual researcher output 1 This paper investigated the use of aggregations functions, in particular the Choquet integral, in a real world application, the analysis of journal rankings and citation data. We have performed a comprehensive analysis of ISI Web of Knowledge citation data in the context of ranking journals for the ERA exercise in Australia (the disciplines of Mathematics, Physics and Computer Science). We have used a number of competing ordinal and non-ordinal classification methods in order to predict the ranking of a journal based on ISI citation data. Our initial hypothesis was that citation data would be an accurate predictor of journal ranking. Our conclusions fall into two categories.. Competitiveness of Choquet integral based classification Our analysis confirms that various competing classification methods have comparable accuracy around 55% on the citation data sets we considered, although for some subsets the accuracy was as good as 75%. The Choquet integral based method fared very well, and delivered the best accuracy on average. Fitting a general fuzzy measure was in general better than fitting WAM, OWA or a single statistic. It supports the fact that various statistics are in fact correlated (redundant or complimentary).. Furthermore, as opposed to other competing methods, the use of the Choquet integral allows one to interpret the resulting Shapley and interaction indices. We found that contrary to our initial assumption, the IF statistic did not have the highest importance, and was in fact less important than 5 year IF, article half-life and article influence. Further, it turned out that IF and 5IF did not show great redundancy, and on average were slightly complementary. The use of Choquet integral also allowed us to identify and have a close look at outliers.. The role of citation data First, we should note that the rankings produced by the ARC are far from being based on either one single statistic, such as IF, or their combination. Our analysis confirms that in fact the ranking process has had a significant "human" dimension, and parameters such as the prestige of a journal and its Editorial Board composition played a role. We noticed the lack of monotonicity in the data, which means that in some cases journals with all citation statistics being higher, received a lower ranking.. We found though, that in general the rankings loosely followed the citation statistic pattern, but not one single statistic in particular. A combination of at least four, or all seven statistics was necessary to produce models which accurately predicted the ranking to within one class.. We found a large variability in the performance of various classification methods on different subsets of the data which correspond to different (but related) sub-disciplines. We found variability in the relative importance and interactions between the variables across these subsets. Generally, citation half-life and article influence were important parameters, but for some subdisciplines IF, 5IF and ii either did or did not play a role. We think that this justifies the introduction of the new statistics such as EF and AI in the ISI data base, despite the fact that they are closely related to IF. When we used four out of seven statistics, we found that 5IF and AI played a greater role in predicting the ranking than the traditional IF, and that EF also featured prominently. Thus the choice of citation statistics should be considered in the context of a specific discipline.. As a general conclusion we would like to state that ranking of journals, even for the purposes of a specific exercise, is a highly complex process that cannot be automated based on citation statistics. The ARC has invested more than three years and thousands of man-hours into this process, and de-livered a list significantly more comprehensive and more accurately reflecting journals' standing than rankings based on one or more citation statistics.. On the other hand, given the large number of journals to rank, part of the ranking process inevitably has to be automated. Citation data, aggregated using such tools as the Choquet integral, can give a valuable starting point for subsequent adjustments. It also allows one to identify and have a close look at potential outliers. If the journal ranking exercise is to be performed on a regular basis, then there is even more room for semi-automatic ranking tools such as the Choquet integral based classifier or other aggregation methods. The wide use of citation data in universities and publication houses to measure the importance of journals and researchers makes it especially important that these statistics and the limits of their interpretations is understood. It is repeated time and again in the academic community that one statistic such as impact factor cannot capture the standing of a journal, although it can be a useful tool. The introduction of other statistics (sometimes in an attempt to find "fairer" measures) increases the amount of information accessible to decision and policy makers, and it seems that, if aggregated reasonably, a rough approximate for the quality of journal can be obtained. The use of more and varied statistics is likely to produce better results than just one or two, however it is obvious that there will be indicators of journal quality that cannot be measured by statistics.. The finding that for this data set, the Choquet integral was competitive and in fact outperformed many ordinal and non-ordinal classification models might not have been anticipated, since it is less flexible than other classifiers such as decision trees. However, it seems that the properties which make the Choquet integral appealing from an interpretive point of view, i.e. its monotonicity, idempotency, piecewise continuity, were actually what allowed it to be robust and consistent when predicting unknown data points. Whilst the Choquet integral is more complicated than a linear model, the Shapley values and interaction indices provide meaningful information that can be used to interpret the resulting model behavior.. The complexity of Choquet integral fitting does increase exponentially with dimension, however, so there will clearly be many situations where its use is infeasible even if it might be theoretically appealing. In future research, it might be worth investigating the classification ability of the Choquet integral on other ordinal and monotone data sets such as those in the UCI repository, some of which are quite high in dimension.. For the ordinal classification method of Frank and Hall, our findings support those of [8] , that ordinal techniques might only marginally improve the accuracy. This is not to say that the use of ordinal extensions to classifiers is unjustified. Where it is known that the class labels are ordinal, taking this information into account in the classification process allows for a more consistent interpretation of the results on top of any accuracy improvements.

O O O O O O O O O B I B I B I O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


In Silico Evolution of Early Metabolism Metabolic evolution, genotype-phenotype map, multi-scale model, evolution of catalysis A version of this paper with color figures is available online at http://dx.doi.org/10.1162/ artl_a_00021. Subscription required.Abstract We developed a simulation tool for investigating the evolution of early metabolism, allowing us to speculate on the formation of metabolic pathways from catalyzed chemical reactions and on the development of their characteristic properties. Our model consists of a protocellular entity with a simple RNA-based genetic system and an evolving metabolism of catalytically active ribozymes that manipulate a rich underlying chemistry.Ensuring an almost open-ended and fairly realistic simulation is crucial for understanding the first steps in metabolic evolution. We show here how our simulation tool can be helpful in arguing for or against hypotheses on the evolution of metabolic pathways. We demonstrate that seemingly mutually exclusive hypotheses may well be compatible when we take into account that different processes dominate different phases in the evolution of a metabolic system. Our results suggest that forward evolution shapes the metabolic networks in the very early steps of evolution. In later and more complex stages, enzyme recruitment supersedes forward evolution, keeping a core set of pathways from the early phase. Understanding the evolutionary mechanisms of complex biological systems is an intriguing and important task of current research in biology as well as artificial life. The mechanisms that governed the formation of metabolic pathways from chemical reactions have been discussed for decades, and several hypotheses have been proposed since the 1940s. Research on the TIM h/a-barrel fold architecture [6] , for instance, shows that the evolution of modern metabolism is mainly driven by enzyme recruitment, as suggested by the patchwork model [31, 55] ). Gene duplications, on the other hand, may facilitate the specialization of an originally multifunctional enzyme, such as carbamoyl phosphate synthetase, to diverse functions in new pathways [27] . Similarly, an entire metabolic pathway may duplicate and specialize, as has been shown to be the case for tryptophan and histidine biosynthesis [18, 31] . The ability of enzymes to catalyze reactions other than those for which they are physiologically specialized, dubbed enzyme promiscuity [33] , forms an important evolutionary reservoir from which novel catalytic functions can be drawn. Promiscuous enzyme activities, although far less efficient than wellevolved ones, can be assembled into novel metabolic pathways [34] , which can provide a selective advantage in particular environmental niches. The evolutionary potential of enzyme promiscuity thus extends far beyond mere enzyme recruitment. Based on such evidence from modern metabolic networks, several hypotheses to explain the evolution of metabolism in general and the emergence of specific metabolic pathways have been suggested. The four most widely cited scenarios are briefly discussed in the following paragraphs. For an in-depth discussion of these theories and further examples of pathway evolution we refer the interested reader to three recent review articles [4, 9, 44] .. The backward evolution hypothesis was one of the first theories of the evolution of metabolic pathways, proposed by Horowitz [26] . It assumes that an organism is able to make use of certain molecules from the environment. However, individuals that can produce these beneficial molecules by themselves gain an advantage in selection in the case of depletion of the food source. Therefore, new chemical reactions are added that produce beneficial molecules from precursors that are abundant in the environment or that are produced in turn by the organismʼs metabolism. As a consequence, one should observe more ancient enzymes downstream in present-day metabolic pathways. Toward the entry point of the pathway, younger and younger enzymes should be found (see Figure 1a) . Backward evolution has been proposed for both the glycolytic pathway [15] and the mandelate pathway [39] .. The forward evolution hypothesis can be seen as an extension or counterpart of the backward evolution hypothesis, reversing the direction of pathway evolution. Granick [19] , and later Cordon [8] , argued for a pathway evolution in the forward direction, requiring that the intermediates be already beneficial to the organism. This is in particular plausible for catabolic pathways, where the organism can extract more energy by breaking food molecules down to simpler and simpler end products. Older enzymes are then expected to be upstream in the pathway, with younger enzymes appearing further downstream (see Figure 1b) . The isoprene lipid pathway [38] is an example for the development of biosynthetic pathways in the forward direction.. The patchwork model [31, 55] explains the formation of pathways by recruiting enzymes from existing pathways. The recruited enzymes may change their reaction chemistry and metabolic function in the new pathways and specialize later through evolution. This introduction of new catalytic activities leads to a selective advantage. Looking at the constitution of a pathway formed by enzyme recruitment, we should observe a mosaic-like picture of older and younger enzymes mixed throughout the pathway (see Figure 1c) . The observation that the TIM h/a-barrel fold architecture occurs in many different pathways corroborates widespread enzyme recruitment in modern metabolism [4] . Other examples are pyrimidine metabolism and histidine biosynthesis [39] .. The shell hypothesis was proposed by Morowitz [35] . It argues, for the case of the reductive citric acid cycle, that in the beginning an autocatalytic core is formed from which new catalytic activities and pathways can be recruited and fed. Thus a metabolic shell would form around this core. Enzymes in the core would likely be less prone to mutational changes, because they are essential for the organism. Thus, one should still be able to observe a core of ancient enzymes (see Figure 1d ). According to Morowitz [35] , the reductive citric acid cycle constitutes such an autotrophic synthetic system.. Each of these mechanisms, thus, is supported by evidence for certain pathways, so that none of these mechanisms is exclusive. Studies on hypotheses of pathway evolution [4, 35] suggest that metabolism has evolved differently in different phases. Furthermore, only traces, or "shadows," are still observable from the events in the very distant past of terrestrial life. Many aspects of the evolutionary history are therefore still not well understood. In particular, the first steps that lead to the emergence of the earliest forms of metabolism evade observation by conventional approaches.. Thus, there is an urgent need for detailed and realistic models of early metabolism that consider all its components and scales. Simulation approaches have been shown to be useful in finding and challenging explanations for the evolution of biological networks [40] . We have recently proposed a computational framework for the evolution of metabolism [12] , modeling all its significant components in a realistic way. Here, we focus on the detailed analysis of evolutionary transitions, aiming in particular at an understanding of the processes underlying metabolic innovation. We have introduced a simulation tool that models the early evolution of metabolism in a quite realistic setting and provides many tools for the detailed investigation of metabolic evolution. The evaluation of the simulation data required support by a visualization framework that facilitates the exploration of the data from different points of view and with different levels of detail. The evolution of a network is expressed in the dynamics of a changing graph by the node connectivity and the attribute values associated with nodes and edges, respectively. In our visual analysis approach, we followed Ben Shneidermanʼs mantra of information visualization: Overview first, zoom and filter, details on demand [47] . The visual exploration of the simulation data thus follows a top-down approach, which has been proven useful when exploring multilevel and multidimensional data. Summarizing time-dependent statistics and lifetime diagrams (e.g., see Figure 8 ) depicting the temporary presence of nodes in the graph helped to gain insight into the topological dynamics of the evolving metabolic network and eventually allowed the visual identification of "interesting" points in time. For these selected situations, evolutionary transitions in pathways can be analyzed in depth, down to the detailed level of exploring the structure and history of individual metabolic pathways and clusters of pathways involving a particular metabolite. We learned that interactivity plays a crucial role in the exploratory phase of the data analysis, due to the multiscale nature of our simulation framework. It was successfully implemented using the linked view method for intuitive navigation in time as well as within a selected network configuration [42] . We can conclude that efficient visualization is an indispensable tool for exploration and hypothesis formulation in complex simulations of molecular processes in general; see, for example [20, 24, 30, 42] , for related applications.. Using both simple examples and a series of more complex simulation runs, the evolution of the components on the small scale (metabolites, enzymes) as well as on systems (pathways, networks) was investigated. The analysis of the genesʼ histories showed all different kinds of evolutionary events, such as convergence, duplication, and divergence, and many different functional transitions from one gene/ enzyme to another, increasing the substrate specificity or changing the reaction chemistry. The simulations further allow us to discriminate between different scenarios for the evolution of metabolic pathways. Based on the observations from this study, we argue that the different evolutionary hypotheses can be reconciled, in that they act in different phases of evolution, so that in different scenarios we might observe another strategy at work. Here, we suggest that forward evolution dominates in the earliest steps and is then superseded by a phase of enzyme recruitment-however, leaving behind a trace in the form of a core set of forward-evolved pathways. We also find that the depletion of important food metabolites introduces backward-evolved pathways. However, the formation is driven by enzyme recruitment rather than being a formation from scratch according to the retrograde hypothesis.. To further test these and other hypotheses on the evolution and formation of metabolic pathways, we intend to simulate a number of different scenarios with changing parameters (mutation rate, duplication rate), define other goals for the organisms (production of one specific metabolite, biomass, or energy), and increase the complexity of the simulation runs (length and number of input molecules).. Albeit our simulation environment is still a drastic simplification of chemistry, it is realistic enough to investigate the evolution of early metabolism. Detailed computer simulations, such as the ones reported here, are likely to provide new insights into the general evolutionary mechanisms governing biological systems, in particular in regimes that are not readily observable. Our approach of a realistic, yet computationally feasible, model appears to be a promising step in this direction.

O O O O O O B I B I B I B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Geographical perspectives on location for location based services In this position paper the question of how location is employed in location based services (LBS) is considered. The importance of the notion of location is highlighted as a means of blurring the boundary between forms of experiences that are direct, and sensed in the environment, and those that are indirect, and learned from information. It is suggested that current methods for modeling location are limited by their lack of strong theoretical underpinning. To help bridge this gap the notions of Space, Place, and Region, from geographical theory, are proposed and implications of these for considering location in LBS outlined. Bringing technology closer to people"s apprehension of the world has long been a goal of contextual and pervasive computing in its various forms. Location based services (LBS) are an example of such a technology, which allow representations of the world to be brought side-by-side with individuals" engagement with it [1] .. Location in such services is emphasised as both a means to scope contents geographically, and a way for information to be presented in context relevant ways. However, location has largely inherited a viewpoint from geo-spatial technologies that treats it as something objective, universal, static, and abstract. Whilst such a treatment provides a detachment appropriate for analysing geographic data, for LBS where the context is people and their direct engagement with the world, this position is far less tenable.. Location is a fundamentally ambiguous term [3] . It refers not only to the continuously changing position of a person, but also their relation to the places, things and other people that interweave through space, time and scale. As such, location cannot easily be separated from the context in which it is made meaningful. Hence, location can be seen as combining both "ego-centric" [2] and geographic factors making its definition problematic.. These observations suggest a theoretical gap in how location can best be presented and represented in LBS. This position paper attempts to navigate this gap by considering location in the light of geographic theory.

O O O O O B I I O O O O O O O O O O O O B I I B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O B I I B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O


Learning a Factor Model via Regularized PCA We consider the problem of learning a linear factor model with an unknown number of factors. We propose a regularized form of principal component analysis (PCA) and demonstrate through experiments with synthetic and real data the superiority of resulting estimates to those produced by pre-existing factor analysis approaches. We also establish theoretical results that explain how our algorithm corrects the biases induced by conventional approaches. An important feature of our algorithm is its computational efficiency being close to that of PCA, which enjoys wide use in large part due to its efficiency. Linear factor models have been widely used for a long time and with notable success in economics, finance, medicine, psychology, and various other natural and social sciences (Harman, 1976) . In such a model, each observed variable is a linear combination of unobserved common factors plus idiosyncratic noise, and the collection of random variables is jointly Gaussian. We consider in this paper the problem of learning a factor model from a training set of vector observations. In particular, our learning problem entails simultaneously estimating the number of factors, the loadings of each factor, and the residual variance of each variable. We seek an estimate of these parameters that best explains out-of-sample data. For this purpose, we consider the likelihood of test data that is independent of the training data. As such, our goal is to design a learning algorithm that maximizes the likelihood of a test set that is not used in the learning process.. A common approach to factor model learning involves application of principal component analysis (PCA) . If the number of factors is known and residual variances are assumed to be uniform, PCA can be applied to efficiently compute model parameters that maximize likelihood of the training data (Tipping and Bishop, 1999) . In order to simplify analysis, we begin our study with a context for which PCA is ideally suited. In particular, before treating more general models, we will restrict attention to models in which residual variances are uniform. As a baseline among learning algorithms, we consider applying PCA together with cross-validation, computing likelihood-maximizing parameters for different numbers of factors and selecting the number of factors that maximizes likelihood of a portion of the training data that is reserved for validation. We will refer to this baseline as uniform-residual rank-constrained maximum-likelihood (URM) estimation.. To improve on URM, we propose uniform-residual trace-penalized maximum-likelihood (UTM) estimation. Rather than estimating parameters of a model with a fixed number of factors and iterating over the number of factors, this approach maximizes likelihood across models without restricting the number of factors but instead penalizing the trace of a matrix derived from the model's covariance matrix. This trace penalty serves to regularize the model and naturally selects a parsimonious set of factors. The coefficient of this penalty is chosen via cross-validation, similarly with the way in which the number of factors is selected by URM. Through a computational study using synthetic data, we demonstrate that UTM results in better estimates than URM. In particular, we find that UTM requires as little as two-thirds of the quantity of data used by URM to match its performance. Further, leveraging recent work on random matrix theory, we establish theoretical results that explain how UTM corrects the biases induced by URM.. We then extend UTM to address the more general and practically relevant learning problem in which residual variances are not assumed to be uniform. To evaluate the resulting algorithm, which we refer to as scaled trace-penalized maximum-likelihood (STM) estimation, we carry out experiments using both synthetic data and real stock price data. The computational results demonstrate that STM leads to more accurate estimates than alternatives available from prior art. We also provide an analysis to illustrate how these alternatives can suffer from biases in this nonuniform residual variance setting.. Aside from the aforementioned empirical and theoretical analyses, an important contribution of this paper is in the design of algorithms that make UTM and STM efficient. The UTM approach is formulated as a convex semidefinite program (SDP), which can be solved by existing algorithms such as interior-point methods or alternating direction method of multipliers (see, e.g., Boyd et al. (2011) ). However, when data dimension is large, as is the case in many contexts of practical import, such algorithms can take too long to be practically useful. This exemplifies a recurring obstacle that arises in the use of SDP formulations to study large data sets. We propose an algorithm based on PCA that solves UTM formulation efficiently. In particular, we found this method to typically require three orders of magnitude less compute time than alternating direction method of multipliers. Variations of PCA such as URM have enjoyed wide use to a large extent because of their efficiency, and the computation time required for UTM is essentially the same as that of URM. STM requires additional computation but remains in close reach for problems where the computational costs of URM are acceptable.. Our formulation is related to that of Chandrasekaran et al. (2010) , which estimates a factor model using a similar trace penalty. There are some important differences, however, that distinguish our work. First, the analysis of Chandrasekaran et al. (2010) focuses on establishing perfect recovery of structure in an asymptotic regime, whereas our work makes the point that this trace penalty reduces nonasymptotic bias. Second, our approach to dealing with nonuniform residual variances is distinctive and we demonstrate through computational and theoretical analysis that this difference reduces bias. Third, Chandrasekaran et al. (2010) treats the problem as a semidefinite program, whose solution is often computationally demanding when data dimension is large. We provide an algorithm based on PCA that efficiently solves our problem. The algorithm can also be adapted to solve the formulation of Chandrasekaran et al. (2010) , though that is not the aim of our paper.. In addition, there is another thread of research on regularized maximum-likelihood estimation for covariance matrices that relates loosely to this paper. Along this line, Banerjee et al. (2008) regularizes maximumlikelihood estimation by the ℓ 1 norm of the inverse covariance matrix in order to recover a sparse graphical model. An efficient algorithm called graphical Lasso was then proposed by Friedman et al. (2008) for solving this formulation. Similar formulations can also be found in Yuan and Lin (2007) and Ravikumar et al. (2011) , who instead penalize the ℓ 1 norm of off-diagonal elements of the inverse covariance matrix when computing maximum-likelihood estimates. Although our approach shares some of the spirit reflected in this line of research in that we also regularize maximum-likelihood estimation by an ℓ 1 -like penalty, the settings are fundamentally different: while ours focuses on a factor model, theirs are based on sparse graphical models. We propose an approach that corrects the bias induced by conventional factor analysis, whereas their results are mainly concerned with accurate recovery of the topology of an underlying graph. As such, their work does not address biases in covariance estimates. On the algorithmic front, we develop a simple and efficient solution method that builds on PCA. On the contrary, their algorithms are more complicated and computationally demanding. We propose factor model estimates UTM and STM, both of which are regularized versions of those that would be produced via PCA. UTM deals with the context where residual variances are assumed to be uniform, whereas STM handles variation among residual variances. Our algorithm for computing the UTM estimate is as efficient as conventional PCA. For STM, we provide an iterative algorithm with guaranteed convergence. Computational experiments involving both synthetic and real data demonstrate that the estimates produced by our approach are significantly more accurate than those produced by pre-existing methods. Further, we provide a theoretical analysis that elucidates the way in which UTM and STM corrects the biases induced by alternative approaches.. Let us close by mentioning a few directions for potential extensions. We have used out-of-sample loglikelihood to assess accuracy of estimates. In practice, estimates are used to guide subsequent decisions. It would be interesting to study the impact of STM on decision quality and whether there are other approaches that fare better in this dimension. Further, in some applications, PCA is used to identify a subspace for dimension reduction. It would be interesting to understand if and when the subspace identified by STM is more suitable. Finally, there is a growing body of research on robust variations of factor analysis and PCA. These include the pursuit of sparse factor loadings (Jolliffe et al., 2003; Zou et al., 2004; D'Aspremont et al., 2004; Johnstone and Lu, 2007; Amini and Wainwright, 2009) , and the methods that are resistant to corrupted data (Pison et al., 2003; Candès et al., 2009; Xu et al., 2010) . It would be interesting to explore connections to this body of work. In this section, we explain why UTM and STM are expected to outperform alternatives as we have seen in our experimental results. We carried out two sets of experiments to compare the performance of aforementioned algorithms. The first is based on synthetic data, whereas the second uses historical prices of stocks that make up the S&P 500 index.

O O B I O O O O O O O O O O O B I O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Multiple Goals and Ownership Structure: Effects on the Performance of Spanish Savings Banks Spanish savings banks are financial institutions with a wide mission that comprises stakeholders' different goals. Thus, profit maximization is only one of the several goals to be achieved, and the widespread use of cost or profit efficiency as the unique comparative measure among banks may prove to be insufficient in this context. To overcome this problem, we build an overall performance measure for organizations that maximize a wide mission with multiple goals. We distinguish two types of savings banks: those controlled by Public Administrations and savings banks controlled by insiders. Each group implies a different arrangement of goals and differences across their performance indexes. Once we control for technology differences, our results show that insider-controlled savings banks favor those goals related to economic efficiency and universal access; furthermore, they perform better. On the other hand, regional development becomes the most favored goal when Public Administrations enjoy the control. The evaluation of efficiency has received preferential attention in the works concerning the banking sector, as can be seen in Berger and Humphrey (1997) , Berger and Mester (1997) and Berger et al. (1999) . A prominent line of investigation analyzes the existing link between organizational forms and efficiency of financial institutions : Aly et al. (1990) , Elyasiani and Mehdian (1992) , Cebenoyan et al. (1993) , Grabowski et al. (1993) , Mester (1993 Mester ( , 1996 , Pi and Timme (1993) , Lang and Welzel (1996) , Mahajan et al. (1996) and Altunbas et al. (2001) . These papers analyze the concern for agency conflict, wondering about which organizational structure works better to reduce costs or to increase profits, although they do not obtain very conclusive evidence in this respect.. Parallel to this, and from a corporate governance perspective, Prowse (1997) indicates that the financial sector is subject to an extensive regulation that reduces the ability of the corporate control market to discipline the managers. The solution to the agency problem comes, thus, from a competitive functioning of financial product markets, but also from the capacity of the different internal control mechanisms, such as ownership structure or the board of directors. Pi and Timme (1993) and Altunbas et al. (2001) investigate the effect that the differences in ownership structure exert on efficiency and, judging by the results, the efficiency of banks is positively related to private ownership of banks (Altunbas et al., 2001) and negatively related to the coincidence, in one person, of the functions of CEO and chairman of the board of directors. Besides, the concentration of ownership, the presence of institutional investors or the proportion of insiders in the board are not significant (Pi and Timme, 1993) .. Regarding the role that financial product markets play in the resolution of the agency conflict, the study of the link between regulation of the above mentioned markets and efficiency of the financial institutions has deserved special attention in the literature; e.g., Kumbhakar et al. (2001) . Thus, some investigations evaluate whether deregulation of credit markets (Bhattacharyya et al., 1997) or deposit markets (Elyasiani and Mehdian, 1995) has had a positive influence on efficiency. In all these works there underlies the need to eliminate obstacles for free competition in the banking sector, as it supposes an increase in competition in the product-market segments that, in the end, favors a better use of the productive resources on the part of the bank managers.. In the Spanish Banking Sector, we observe also an increasing deregulation, which has affected more prominently the savings banks, historically subject to major restrictions than commercial banking (Kumbhakar et al., 2001) . Nowadays, the activity of savings banks has been equalized to that of banks and savings banks have achieved remarkable increases in productivity (Grifell-Tatjé and Lovell, 1997) . When the operating system of both types of banks becomes similar, public authorities have centered their efforts to improve the efficiency of savings banks through the reform of the composition and functioning of their government bodies (Law 44/2002 , of November 22, 2002 .. Nevertheless, in order to evaluate the effect of this and other laws on savings banks management, it is not sufficient to evaluate efficiency in profits or in costs. The legislator has assigned to savings banks an important set of goals -some of them in contradiction with the maximization of value-at the same time that it has allocated the ownership of savings banks to several groups of stakeholders.. Considering what has previously been stated, the present article aims at fulfilling a double objective. First, we consider the elaboration of an aggregate index of performance to evaluate the management of organizations with a multiple-goal mission.. This index would become a way to calibrate the consequences of the possible conflict between agent and principals. Secondly, we present an application of the methodological proposal to the Spanish savings banks sector, characterized by the presence of several stakeholders and a multiplicity of goals, on the one hand, but also by the presence of a legislator, who establishes his preferences, thus influencing on the composition of the entities' government bodies.. The elaboration of an aggregate index of performance raises remarkable technical and empirical difficulties. First, in order to carry out an aggregation of goals it is necessary to know the weights attached to each goal, and this calculation is far from being trivial. Secondly, the production function of the above-mentioned goals is not known. The Data Envelopment Analysis (DEA) technology arose precisely to give response to some of these difficulties, as described in Lovell et al. (1995) . The identification of good practices in the management of multiple-goal organizations has already been treated in the literature on efficient frontiers. Thus, Bendheim et al. (1998) use DEA methodology to construct an indicator of corporate social responsibility towards the principal stakeholders, Piesse and Townsend (1995) evaluate the efficiency of building societies in the United Kingdom and Mester (1993) applies it to the Savings and Loans in the United States. This empirical approach is also usual in other contexts, as in the case of Lovell (1995) and Lovell et al. (1995) , who elaborate an aggregate index of macroeconomic performance for countries from individual indicators such as price stability, growth, level of employment and commercial scale.. Following this line of research, our work approaches the evaluation of the efficiency of Spanish savings banks from an alternative perspective. A common feature in most of the works that evaluate savings banks management is the use of the same indicators as for the case of private banking. That is, by means of productivity (Pastor, 1995, and Grifell-Tatjé and , costs (Maudos, 1996 , Lozano-Vivas, 1998 and Tortosa-Ausina, 2003 ), or profits (Lozano-Vivas, 1997 . Furthermore, these works compare the results obtained in terms of the above-mentioned indicators, wonderingimplicitly or explicitly-if the differences of ownership and government structure between banks and savings banks may cause different levels of efficiency. Nevertheless, the legislation has assigned a wider mission to savings banks since it has conformed a set of goals for these financial institutions, at the same time that it has assigned their ownership to several groups of stakeholders. According to this perspective, the managers of savings banks should replace the goal of value maximization with that of the maximization of the aggregation of goals included in the mission. Our work incorporates this multiplicity of goals and, in addition, it includes an analysis of the preferences of the legislator, proposing a methodology that allows us to evaluate the effects of changes in the configuration of the government bodies on the management and efficiency of savings banks.. In order to reach this aim, we first describe the frame in which the agency problem between the savings bank agent and the legislator takes place, based on in force legislation. Secondly, we identify differences in ownership structure and, therefore, in the mission of savings banks, thus making it necessary to control these differences in productive technology. Thirdly, we evaluate managers according to their ability to satisfy the above-mentioned goals; for this purpose we propose a first index of performance.. Fourthly, we contribute to define the structure of the mission by identifying the weights associated to each of the savings banks' goals. Finally, we incorporate the legislator's preferences, identified by the existing legislation on the government bodies, and we elaborate a second index of performance. Thus, it is possible to evaluate managers in the frame of a new contract that regulates the agency problem and that now turns out to be more complete (in the sense of more restricted). In this context, the efficiency of a savings bank will depend on the decisions made by the agent (as it happened in the first index), but also on the agent's ability to correctly understand the legislator's preferences.. The article is structured as follows. In Section 2, we review briefly the history of savings banks in order to define their mission and to identify stakeholders. The relationship between the methodology used and the underlying economic problem is presented in Section 3. The data and variables used in the empirical application are described in Section 4; while the results of this application appear in Section 5. The paper ends with a section of conclusions. In recent works, among which Tirole (2001) should be outlined, a firm is considered a complex organization, where the search for particular interests does not necessarily lead to the maximization of social efficiency due to the presence of external factors that prevent it (e.g., hold-up and quasi-rents linked to the presence of specific investments). This is the reason why we have proposed to characterize a firm as an economy with presence of external effects, (Holmström, 1999) . According to this approach, managers should replace the goal of maximizing shareholder value with that of the maximization of an aggregation of stakeholders' value. By extension, the concern about the good government of the firm would center then on the design of institutions that force managers to recognize this new mission. In this context, the construction of an aggregate index of performance represents a way to calibrate the consequences of the possible agency conflict between the agent, the firm's managing team, and the multiple principals, that is, the stakeholders. At the same time, this index allows us to evaluate the contribution of each possible government alternative to solve the agency problem.. The aim of this work has been to study the particular management system of Spanish SB, identifying the degree in which different ownership structures contribute to the attainment of the goals marked by the mission of these financial institutions.. Reviewing the historical procedure and in-force laws has allowed us to identify the two dimensions that define the management system of SB: a wide mission that puts together the interests of such stakeholders as the founder entity, the PA, the depositors and the regulatory authorities, and a control system close to the model of shared control, since in the government bodies also different groups take part (founder entity, PA, contributors and personnel). Both characteristics turn the traditional analysis of SB government into an inadequate one from the perspective of the agency problem between managers and shareholders praised by Shleifer and Vishny (1997) and it points out the need for a efficiency measure that allows us to evaluate the contribution of managers to the maximization of a multiple-goal mission.. In this paper we propose the use of Data Envelopment Analysis (DEA). DEA presents remarkable advantages, as it allows getting used to situations of multiplicity of inputs and outputs, it requires less information about the underlying technology, and it solves the problem of aggregation when the weights attached to each variable are not available. This technique also allows us to elaborate an index of performance in the attainment of the mission and to obtain the weights attached to each of its goals. These weights can be interpreted as the relative importance of goals, that is, the result of negotiation among stakeholders according to previously defined management rules and to a specific allocation of control rights. Nevertheless, occasionally the legislator interferes in the negotiation among agents, trying to impose his own preferences, for example, by means of the regulation of the composition of the government bodies, and a question arises as to whether regulation has managed to implement the legislator's preferences in the organization. In the answer to this question, DEA allows us to externally impose restrictions on the weights included in the legislator's preferences.. Thus, it is possible to evaluate the distance that separates current decisions from those decisions that would place it on the frontier of good practices defined by the legislator.. Using the present European legislation, our empirical analysis differentiates two models of savings banks: those controlled by insiders and those controlled by PA. The differences in productive specialization and organizational characteristics of both types of banks have made it necessary for us to control for the productive technology as well.. In order to evaluate managers, the index of performance resulting from the incorporation of the legislator's preferences provides us with a measurement of the overall efficiency reached by the savings bank, which we have decomposed into technical efficiency (the index of performance without restrictions) and allocative efficiency.. When differentiating savings banks by their control (insider or public), we have confirmed that the differences in performance with revealed preferences are not explained by managers' mistakes when interpreting the legislator's preferences, since allocative inefficiency is, practically, insignificant. We do confirm, however, the existence of substantial differences in technical efficiency, understood as the managers' skill to transform inputs into goals for the stakeholders. And it is precisely these differences that If we center our attention on the debate about the government of savings banks and, especially, on the inverse relation between over-representation of some groups in the government bodies and economic efficiency reached by SB, we can argue that, in the light of the results obtained, the differences in ownership structure give rise to differences in SB technology and priority order, including the goal of economic efficiency. This is the reason why savings banks should not be evaluated exclusively in terms of economic efficiency, since they pursue multiple and different goals, which presumably law arises is whether it will improve SB efficiency. In view of our results, we can argue that the decrease in the weight of the PA group itself would suppose a change in the structure of priorities of the entity (i.e., it would increase the weight of goal "economic efficiency" to the detriment of other goals, as "regional development") and a light increase in the overall index of performance (our RIP). Nevertheless, global inefficiency would still be considerable (around 10 per cent). In this respect, one must only wait for the ending of the events to see if these changes suppose an improvement of technical efficiency, which probably constitutes the main problem of Spanish savings banks.

O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Introducing Assignment Functions to Bayesian Optimization Algorithms In this paper we improve Bayesian optimization algorithms by introducing proportionate and rank-based assignment functions. A Bayesian optimization algorithm builds a Bayesian network from a selected sub-population of promising solutions, and this probabilistic model is employed to generate the offspring of the next generation. Our method assigns each solution a relative significance based on its fitness, and this information is used in building the Bayesian network model. These assignment functions can improve the quality of the model without performing an explicit selection on the population. Numerical experiments demonstrate the effectiveness of this method compared to a conventional BOA. In order to realize a truly robust search algorithm, one that is applicable to a wide spectrum of applications, a variety of evolutionary algorithms have been proposed. Genetic algorithms (GAs) maintain a population of solution candidates in the problem domain, each solution represented by a parameter string, and arrive at an optimal or near-optimal solution by applying selection, recombination, and mutation operators over repeated generations.. Email addresses: munetomo@iic.hokudai.ac.jp (Masaharu Munetomo), naoya.m@cims.hokudai.ac.jp (Naoya Murao), akama@cims.hokudai.ac.jp (Kiyoshi Akama).. It is essential to ensure tight linkage for effective genetic search through recombination operators such as crossovers. When a subset of loci in the parameter space is found to form a building block (BB) -a good candidate for a subsolution of the larger problem -these parameters should be "tightly" encoded on a string. In other words, they should be closely linked to each other in such a way that recombination operations will preserve the BB when a new generation of solutions is created. Conventional recombination operators such as one-point crossover will easily disrupt BBs if they are not tightly linked on a string.. In the early history of GA applications, it was common to employ problemspecific knowledge to ensure the tight linkage of associated parameters. This advance knowledge is not always available when starting a new genetic search, so it is sometimes difficult to ensure tight linkage by this approach.. More advanced GAs have recently been proposed that can learn or identify linkage groups, either directly or indirectly, without the need for problemspecific knowledge. The "Linkage Learning" GA, for example, applies twopoint crossovers on circular strings to obtain indirect information on important linkages. The Linkage Identification with Nonlinearity Check (LINC) algorithm [12] , on the other hand, pioneered a number of techniques for directly identifying linkage groups by detecting nonlinear behavior under pair-wise perturbations.. In addition to these linkage identification techniques, estimation of distribution algorithms (EDAs) have been introduced that obtain linkage information indirectly by estimating the probable distribution of alleles in a set of promising solutions. [13] Unlike classical GAs, EDAs do not apply genetic operators to individual candidate solutions. Instead they generate offspring based on the estimated probabilistic distribution of a population of promising solutions. Linkage information is implicit in these distributions, so we do not need to ensure tight linkage in advance.. In this paper, we propose a novel approach of EDAs employing assignment functions in the estimation process of Bayesian optimization algorithm (BOA) [15] [16] [17] 19, 20] , one of the most sophisticated approach in EDAs. The BOA first selects a set of promising solutions from the original population, using the information in these candidates to estimate the probabilistic distribution of the solution space. Aside from this selection step, however, differences in fitness among the population are not taken into account -the chosen solutions are treated equally in building the model distribution. In order to improve the accuracy of the model, one can introduce a function that assigns different relative weights to individual solutions according to their significance. By employing these assignment functions, we can simplify the algorithm, making it unnecessary to perform explicit selections -the BOA simply repeats model-building and generation of solution candidates based on the model. This paper is organized as follows. First we review EDAs briefly, including the BOA proposed by previous studies. Second, we introduce our assignment functions and show how they can improve the model-building accuracy of the BOA. Finally we perform numerical experiments, and compare the effectiveness of the proposed method with conventional approaches. In this paper, we show that assignment functions can result in more accurate probabilistic model-building and improved overall performance for the Bayesian Optimization Algorithm (BOA). The proposed assignment functions are inspired by the processes of proportionate and rank-based reproduction in conventional genetic algorithms. By assigning a greater significance to the best solutions of the current population, we improve the accuracy of the probabilistic models. Our approach also simplifies the conventional BOA by removing the explicit selection process; it simply alternates between building probabilistic models and generating solution candidates based on the models.. Through empirical comparisons, we demonstrate that the assignment function approach is more effective than the conventional BOA. Rank-based assignment functions are a robust choice, improving performance for all the test problems examined here. Proportionate functions achieve better results than rank-based functions for certain uniformly scaled test functions, but achieve no significant improvement for exponentially scaled functions. Parameter optimization is an essential topic for future work, especially in the case of proportionate assignment functions, to improve the robustness of this technique.

O B I O B I I O O O O O B I I O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O


Visual Person Identification Using a Distance-dependent Appearance Model for a Person Following Robot This paper describes a person identification method for a mobile robot which performs specific person following under dynamic complicated environments like a school canteen where many persons exist. We propose a distance-dependent appearance model which is based on scale-invariant feature transform (SIFT) feature. SIFT is a powerful image feature that is invariant to scale and rotation in the image plane and also robust to changes of lighting condition. However, the feature is weak against affine transformations and the identification power will thus be degraded when the pose of a person changes largely. We therefore use a set of images taken from various directions to cope with pose changes. Moreover, the number of SIFT feature matches between the model and an input image will decrease as the person becomes farther away from the camera. Therefore, we also use a distance-dependent threshold. The person following experiment was conducted using an actual mobile robot, and the quality assessment of person identification was performed. There is an increasing demand for service robots operating in public space like a shopping mall. An example of service task is to follow a person who is carrying his/her items. This research develops a person identification method for such a robot that can follow a specific user among obstacles and other people.. There have been a lot of works on person detection and tracking using various image features [1, 2] . HOG [3] is currently one of the most widely used features for visual people detection. The detection methods in consideration of occlusion have also been proposed [4, 5] , but emphasis is put on detection performance rather than the processing speed. Moreover, the person detection methods which combine HOG and the distance information acquired using an RGB-D camera such as Microsoft Kinect sensor were also proposed [6−8] . Spinello and Arras [6] performed an experiment using fixed cameras in the lobby of an university canteen. Munaro et al. [7] showed an example of tracking result using a mobile robot in an exhibition. Kinect sensor, however, cannot be used under sunlight. Ess et al. [9, 10] proposed to integrate various cues such as appearance-based object detection, depth estimation, visual odometry, and ground plane detection using a graphical model for pedestrian detection. Their method exhibits a nice performance for complicated scenes where many pedestrians exist. However, it is still costly to be used for controlling a real robot. Frintrop et al. [11] proposed a visual tracker for mobile platforms, but their experiments were performed in only laboratory environments.. We built a mobile robot system with a stereo camera and a laser range finder [12] , and realized specific person following in a complex environment with several walking people at a time. The method, however, did not have a sufficient performance to recognize people with similar clothing. In real environment where many ordinary people exist, it is important to distinguish the target person from ordinary people who wear various clothing.. Color histogram is widely used for person identification by a mobile robot [13−15] . Zajdel et al. [13] proposed a method in consideration of slow illumination changes by using a local trajectory of color feature. However, when the lighting conditions change, it is difficult to distinguish the person correctly by using the color-based method. The methods of person identification using not only clothes but also face images [14] or gait patterns [16] were proposed. However, the robot system which follows a person from behind cannot often work well with those methods.. In this paper, we propose a method of identifying a person based on the pattern of clothing using the scaleinvariant feature transform (SIFT) feature. We make the appearance model from various body directions, and set a distance-dependent threshold to cope with the decrease of the number of SIFT feature matches due to the increased distance.. The organization of this paper is as follows. We describe our previous tracking system and its problems in Section 2. In Section 3, we propose a SIFT feature-based person identification method. In Section 4, we implement the proposed method on an actual robot to perform person tracking experiments. Finally, we conclude this paper and discuss future work in Section 5. In this paper, we proposed a person identification method using SIFT feature for a mobile robot which performs specific person following. We made the appearance model for various body directions, and set the distance-dependent threshold to cope with the decrease of the number of SIFT feature matches according to the increased distance. Experimental results showed that the proposed method is able to identify the person even when other people with a similar color clothing exist near the target person. Using the method, the robot successfully followed a specific person in the cafeteria.. For more robust identification, it is necessary to additionally use other sensors such as a laser range finder or other personal features such as the height or gait patterns.

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