bluuebunny/crossref_metadata_2025 huggingface dataset:
Detailed analysis of coupling constants and isotope effects in NMR spectra of isotopomers of 12C6813C2
Authors: Anklin Clemens , Alemany Lawrence B.
Published: February 2006

A preliminary study of the long‐range (i.e. two‐bond or longer) 13C13C coupling constants in natural abundance C70 shows, consistent with recent theoretical calculations by Peralta et al. that the largest long‐range JCC values for the polar and equatorial sites are clearly smaller than the largest long‐range JCC values for the other three sites. The unusually large size of the 2JCC couplings between inequivalent carbons in a nonpolar pentagon in C70 has no analog among 2JCC data reported for planar aromatic compounds. No long‐range JCC values appear to have been reported for any curved aromatic compounds. In addition, much more precise 1JCC values were obtained for C70 than was possible about 15 years ago. Comparing the chemical shifts for each of the five isotopomers of C70 containing only one 13C nucleus and the frequencies of the satellites for each of the four isotopomers containing two adjacent and inequivalent 13C nuclei indicates that replacing 12C with 13C shields the adjacent 13C nucleus by 15 to 23 ppb, consistent with the limited 1Δ13C(13/12C) isotope effect data available on a few small aromatic molecules. Such measurements become possible with natural abundance C70 only by using a 13C cryoprobe and a high‐field spectrometer (700 MHz). The additional information that could be obtained from a spectrum obtained under ultrahigh resolution conditions is discussed. Secure identification of the singlets arising from the four 12C6813C2 isotopomers with equivalent adjacent 13C nuclei is necessary to allow the largest long‐range JCC values to be precisely determined. The presence of numerous isotopomers containing two or more 13C nuclei would present a great challenge in interpreting the various signals in a spectrum obtained under ultrahigh resolution conditions. Copyright © 2006 John Wiley & Sons, Ltd.
Chemical shift imaging (CSI) by precise object displacement
Authors: Leclerc Sebastien , Trausch Gregory , Cordier Benoit , Grandclaude Denis , Retournard Alain , Fraissard Jacques , Canet Daniel
Published: February 2006

A mechanical device (NMR lift) has been built to displace vertically an object (typically an NMR sample tube) inside the NMR probe with an accuracy of 1 µm. A series of single pulse experiments are performed for incremented vertical positions of the sample. With a sufficiently spatially selective radio‐frequency (r.f.) field, one obtains chemical shift information along the displacement direction (one‐dimensional chemical shift imaging (CSI)). Knowing the vertical r.f. field profile (the amplitude of the r.f. field along the vertical direction), one can reconstruct the spectrum associated with all the slices corresponding to consecutive sample positions and improve the spatial resolution, which is simply related to the accuracy of the displacement device. Beside tests performed on phantoms, the method has been applied to solvent penetration in polymers and to benzene diffusion in a heterogeneous zeolite medium. Copyright © 2006 John Wiley & Sons, Ltd.
1H relaxation times of metabolites in biological samples obtained with nondestructive ex‐vivo slow‐MAS NMR
Authors: Hu Jian Zhi , Wind Robert A. , Rommereim Donald N.
Published: February 2006

Methods suitable for measuring 1H relaxation times such as T1, T2 and T1ρ of metabolites in small, intact biological objects including live cells, excised organs and tissues, oil seeds etc. are developed in this work. This was achieved by combining inversion‐recovery, spin‐echo, or a spin‐lock segment with the phase‐adjusted spinning sideband (PASS) technique, which was applied at low sample‐spinning rates. Here, PASS was used to produce high‐resolution 1H spectra in a nondestructive way so that the relaxation parameters of individual metabolite could be determined. The methodologies were demonstrated by measuring 1H T1, T2, and T1ρ of metabolites in excised rat liver at a spinning rate of 40 Hz. Copyright © 2006 John Wiley & Sons, Ltd.
On the use of pseudocontact shifts in the structure determination of metalloproteins
Authors: Jensen Malene Ringkjøbing , Hansen D. Flemming , Ayna Umit , Dagil Robert , Hass Mathias A. S. , Christensen Hans E. M. , Led Jens J.
Published: February 2006

The utility of pseudocontact shifts in the structure refinement of metalloproteins has been evaluated using a native, paramagnetic Cu2+ metalloprotein, plastocyanin from Anabaena variabilis (A.v.), as a model protein. First, the possibility of detecting signals of nuclei spatially close to the paramagnetic metal ion is investigated using the WEFT pulse sequence in combination with the conventional TOCSY and 1H–15N HSQC sequences. Second, the importance of the electrical charge of the metal ion for the determination of correct pseudocontact shifts from the obtained chemical shifts is evaluated. Thus, using both the Cu+ plastocyanin and Cd2+‐substituted plastocyanin as the diamagnetic references, it is found that the Cd2+‐substituted protein with the same electrical charge of the metal ion as the paramagnetic Cu2+ plastocyanin provides the most appropriate diamagnetic reference signals. Third, it is found that reliable pseudocontact shifts cannot be obtained from the chemical shifts of the 15N nuclei in plastocyanin, most likely because these shifts are highly dependent on even minor differences in the structure of the paramagnetic and diamagnetic proteins. Finally, the quality of the obtained 1H pseudocontact shifts, as well as the possibility of improving the accuracy of the obtained structure, is demonstrated by incorporating the shifts as restraints in a refinement of the solution structure of A.v. plastocyanin. It is found that incorporation of the pseudocontact shifts enhances the precision of the structure in regions with only few NOE restraints and improves the accuracy of the overall structure. Copyright © 2006 John Wiley & Sons, Ltd.
High‐resolution magic‐angle spinning 13C spectroscopy of brain tissue at natural abundance
Authors: Yang YongXia , Chen Lei , Gao HongChang , Zeng DanLin , Yue Yong , Liu MaiLi , Lei Hao , Deng Feng , Ye ChaoHui
Published: February 2006

High‐resolution magic‐angle spinning (MAS) 1H and 13C magnetic resonance spectroscopy (MRS) has recently been applied to study the metabolism in intact biological tissue samples. Because of the low natural abundance and the low gyromagnetic ratio of the 13C nuclei, signal enhancement techniques such as cross‐polarization (CP) and distortionless enhancement by polarization transfer (DEPT) are often employed in MAS 13C MRS to improve the detection sensitivity. In this study, several sensitivity enhancement techniques commonly used in liquid‐ and solid‐state NMR, including CP, DEPT and nuclear Overhauser enhancement (NOE), were combined with MAS to acquire high‐resolution 13C spectra on intact rat brain tissue at natural abundance, and were compared for their performances. The results showed that different signal enhancement techniques are sensitive to different classes of molecules/metabolites, depending on their molecular weights and mobility. DEPT was found to enhance the signals of low–molecular weight metabolites exclusively, while the signals of lipids, which often are associated with membranes and have relatively lower mobility, were highly sensitive to CP enhancement. Copyright © 2006 John Wiley & Sons, Ltd.
Quantitative glutamate spectroscopic imaging of the human hippocampus
Authors: Pan J. W. , Venkatraman T. , Vives K. , Spencer D. D.
Published: February 2006

We have evaluated a three‐dimensional localized spectroscopic imaging sequence that uses two pairs of adiabatic full‐passage pulses, which optimizes the detection of glutamate resonances at moderate echo times. This sequence provides excellent volume localization while simultaneously reducing J‐modulation losses of glutamate. We have simulated the performance of this sequence for glutamate and used it to quantitatively measure glutamate in the human hippocampus using a linear components model. Using tissue segmentation and regression analysis, we measured a glutamate concentration of 8.8 ± 2.1 mM in hippocampal and temporal gray matter and 3.7 ± 1.1 mM in temporal white matter (95% CI). We have used this approach in a small group of patients (n = 5) with unilateral hippocampal epilepsy. Copyright © 2006 John Wiley & Sons, Ltd.
Relative and absolute quantitative expression profiling of cytochromes P450 using isotope‐coded affinity tags
Authors: Jenkins Rosalind E. , Kitteringham Neil R. , Hunter Christie L. , Webb Sally , Hunt Tony J. , Elsby Robert , Watson Rod B. , Williams Dominic , Penningtonand Stephen R. , Park B. Kevin
Published: February 2006

The development of a novel method for absolute quantification of proteins based on isotope‐coded affinity tagging using ICAT® reagents is described. The method exploits synthetic peptide standards to determine protein content at the femtomole level in biological samples. The approach is generally applicable to any subset of proteins, but is particularly appropriate for quantitative analysis of multiple, closely related isoforms, and for hydrophobic proteins that are poorly represented in 2‐D gels. Relative and absolute quantification techniques are applied to an important group of microsomal metabolic enzymes, the cytochromes P450 (P450), which are critical in determining the disposition, safety and efficacy of drugs in man. Measurement of the P450 induction profile in response to chemicals is a fundamental aspect of drug safety evaluation and is currently achieved by low‐throughput methods employing poorly discriminatory antibodies or substrates. Tagging technology is shown to supersede conventional methods for P450 profiling in terms of discriminatory power and throughput, exemplified by the simultaneous detection of distinct induction profiles for cyp2c subfamily members in response to phenobarbitone: cyp2c29 expression, but not cyp2c40 or cyp2c50, was induced threefold by treatment. This technology should abbreviate the drug development pathway, and provide a widely applicable, rapid means of quantifying proteins.
Systematic localisation of proteins fused to the green fluorescent protein in Bacillus subtilis: Identification of new proteins at the DNA replication factory
Authors: Meile Jean‐Christophe , Wu Ling Juan , Ehrlich S. Dusko , Errington Jeff , Noirot Philippe
Published: February 2006

Construction and microscopic imaging of protein fusions to green fluorescent protein (GFP) have revolutionised our understanding of bacterial structure and function. We have undertaken a systematic study of the localisation of over 100 Bacillus subtilis proteins, following the development of high‐throughput construction and analysis procedures. We focused on proteins linked in various ways to the DNA replication machinery, as well as on proteins exemplifying a range of other cellular functions and structures. The results validate the approach as a way of obtaining systematic protein localisation information. They also provide a range of novel biological insights, particularly through the identification of a number of proteins not previously known to be associated with the DNA replication factory.
Synthesis, crystallization kinetics, and spherulitic growth of linear and star‐shaped poly(L‐lactide)s with different numbers of arms
Authors: Wang Lu , Dong Chang‐Ming
Published: February 2006

Well‐defined linear poly(L‐lactide)s with one or two arms (LPLLA and 2LPLLA, respectively) and star‐shaped poly(L‐lactide)s with four or six arms (4sPLLA and 6sPLLA, respectively) were synthesized and then used for the investigation of the thermal properties, isothermal crystallization kinetics, and spherulitic growth. The maximal melting temperature, the cold‐crystallization temperature, and the degree of crystallinity of these poly(L‐lactide) polymers decreased with an increasing number of arms in the macromolecule. Moreover, the isothermal crystallization rate constant (K) of these poly(L‐lactide) polymers decreased in the order of KLPLLA > K2LPLLA > K4sPLLA > K6sPLLA2, which was consistent with the variation trend of the spherulitic growth rate (G). Meanwhile, both K and G of 6sPLLA slightly increased with the increasing molecular weight of the polymer. Furthermore, both LPLLA and 2LPLLA presented spherulites with good morphology and apparent Maltese cross patterns, whereas both unclear Maltese cross patterns and imperfect crystallization were observed for the star‐shaped 4sPLLA and 6sPLLA polymers. These results indicated that both the macromolecular architecture and the molecular weight of the polymer controlled K, G, and the spherulitic morphology of these poly(L‐lactide) polymers. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2226–2236, 2006
A new soluble conducting polymer and its electrochromic devices
Authors: Yildiz Umit Hakan , Sahin Elif , Akhmedov Idris M. , Tanyeli Cihangir , Toppare Levent
Published: February 2006

A new polythiophene derivative was synthesized by both chemical and electrochemical oxidative polymerization of 1‐(1‐phenylethyl)‐2,5‐di(2‐thienyl)‐1H‐pyrrole (PETPy). Of which the chemical method produces a polymer that is completely soluble in organic solvents. The structures of both the monomer and the soluble polymer were elucidated by nuclear magnetic resonance (1H and 13C NMR) and Fourier transform infrared (FTIR) spectroscopy. The average molecular weight has been determined by gel permeation chromatography to be Mn = 3.29 × 103 for the chemically synthesized polymer. Polymer of PETPy was synthesized via potentiostatic electrochemical polymerization in acetonitrile (AN)/NaClO4/LiClO4 (0.1 M) solvent–electrolyte couple. Characterizations of the resulting polymer were performed by cyclic voltammetry, FTIR, scanning electron microscopy, and UV–vis spectroscopy. Four‐probe technique was used to measure the conductivities of the samples. Moreover, the spectroelectrochemical and electrochromic properties of the polymer films were investigated. In addition, dual‐type polymer electrochromic devices based on P(PETPy) with poly(3,4‐ethylenedioxythiophene) were constructed. Spectroelectrochemistry, electrochromic switching, and open circuit stability of the devices were studied. They were found to have good switching times, reasonable contrasts, and optical memories. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2215–2225, 2006
Pediatric Churg‐Strauss syndrome in Mexico
Authors: Hernández‐Bautista Víctor Manuel , Espinosa‐Padilla Sara Elva , Yamazaki‐Nakashimada Marco Antonio , López‐Lara Deyanira , González‐Serrano Edith , Staines‐Boone Tamara , Espinosa‐Rosales Francisco
Published: February 2006

Churg‐Strauss syndrome (CSS) is one of the rarest forms of vasculitis, and very rarely presents in the pediatric population. We present two cases of childhood CSS, both with hepatic and cardiac involvement. To our knowledge, these are the first two cases of CSS in the pediatric population described in Mexico. Pediatr Pulmonol. © 2006 Wiley‐Liss, Inc.
Electrospray ionisation mass spectral studies on hydrolysed products of sulfur mustards
Authors: Lakshmi V. V. S. , Murty M. R. V. S. , Reddy T. Jagadeshwar , Ravikumar M. , Prabhakar S. , Vairamani M.
Published: February 2006

Off‐site detection of the hydrolysed products of sulfur mustards in aqueous samples is an important task in the verification of Chemical Weapons Convention (CWC)‐related chemicals. The hydrolysed products of sulfur mustards are studied under positive and negative electrospray ionisation (ESI) conditions using an additive with a view to detecting them at trace levels. In the presence of cations (Li+, Na+, K+ and NH), the positive ion ESI mass spectra of all the compounds include the corresponding cationised species; however, only the [M+NH4]+ ions form [M+H]+ ions upon decomposition. The tandem mass (MS/MS) spectra of [M+H]+ ions from all the hydrolysed products of the sulfur mustard homologues were distinct and allowed these compounds to be characterised unambiguously. Similarly, the negative ion ESI mass spectra of all the compounds show prominent adducts with added anions (F−, Cl−, Br−, and I−), but the [M–H]− ion can only be generated by decomposition of an [M+F]− ion. The MS/MS spectra of the [M–H]− ions from all the compounds result in a common product ion at m/z 77. A precursor ion scan of m/z 77 is shown to be useful in the rapid screening of these compounds in aqueous samples at trace levels, even in the presence of complex masking agents, without the use of time‐consuming sample preparation and chromatography steps. An MS/MS method developed to measure the detection limits of the hydrolysed products of sulfur mustards found these to be in the range of 10–500 ppb. Copyright © 2006 John Wiley & Sons, Ltd.
Comparison between external and internal standard calibration in the validation of an analytical method for 1‐hydroxypyrene in human urine by high‐performance liquid chromatography/tandem mass spectrometry
Authors: Pigini D. , Cialdella A. M. , Faranda P. , Tranfo G.
Published: February 2006

1‐Hydroxypyrene is a metabolite of pyrene, a member of the class of polycyclic aromatic hydrocarbons (PAHs) whose toxic properties in some cases include carcinogenicity. The determination of 1‐hydroxypyrene in human urine is used as a biological indicator for exposure to PAHs, which is related to the combustion of organic materials, like smoking, living in urban environments, and eating grilled or smoked food. The determination of 1‐hydroxypyrene by high‐performance liquid chromatography (HPLC) with fluorescence detection has very good sensitivity but it is not highly specific: this can reduce accuracy in the quantitative determination of low levels of analyte in a complex matrix like urine. An HPLC method that uses triple quadrupole mass detection has been validated with the objective both to improve the signal‐to‐noise (S/N) ratio and to achieve the maximum specificity for the analyte in those urine samples that are richer in possible inteferents. The calibration range for 1‐hydroxypyrene is from 0.005–0.1 µg/L in the urine of non‐smoking healthy volunteers. After solid‐phase extraction, samples were analyzed by HPLC/tandem mass spectrometry (MS/MS) in the multiple reaction monitoring (MRM) mode. In order to obtain reliable results quantitative analysis must be performed by means of the internal standard method (we used deuterium‐labelled 1‐hydroxypyrene): the method accuracy is not less than 85%. The S/N ratio at a concentration of 0.1 µg/L is about 10, and therefore this can be considered the lowest limit of quantitation. The method performance does not change if urine samples are measured using a calibration curve prepared in methanol, thus reducing the time of analysis and costs. Copyright © 2006 John Wiley & Sons, Ltd.
Almost universal graphs
Authors: Frieze Alan , Krivelevich Michael
Published: February 2006

We study the question as to when a random graph withnvertices andmedges contains a copy of almost all graphs withnvertices andcn/2 edges,cconstant. We identify a “phase transition” atc= 1. Forc< 1,mmust grow slightly faster thann, and we prove thatm=O(nlog logn/log log logn) is sufficient. Whenc> 1,mmust grow at a ratem=n1+a, wherea=a(c) > 0 for everyc> 1, anda(c) is between$1-{2 \over (1+o(1))c}$and$1-{1 \over c}$for large enoughc. © 2006 Wiley Periodicals, Inc. Random Struct. Alg., 2006
An Estimating Equations Approach for the LISCOMP Model
Authors: Reboussin Beth A. , Liang Kung-Yee
Published: February 2006

Maximum likelihood estimation is computationally infeasible for latent variable models involving multivariate categorical responses, in particular for the LISCOMP model. A three-stage generalized least squares approach introduced by Muthén (1983, 1984) can experience problems of instability, bias, non-convergence, and non-positive definiteness of weight matrices in situations of low prevalence, small sample size and large numbers of observed indicator variables. We propose a quadratic estimating equations approach that only requires specification of the first two moments. By performing simultaneous estimation of parameters, this method does not encounter the problems mentioned above and experiences gains in efficiency. Methods are compared through a numerical study and an application to a study of life-events and neurotic illness.
Nonparametric Polytomous IRT Models for Invariant Item Ordering, with Results for Parametric Models
Authors: Sijtsma Klaas , Hemker Bas T.
Published: February 2006

It is often considered desirable to have the same ordering of the items by difficulty across different levels of the trait or ability. Such an ordering is an invariant item ordering (IIO). An IIO facilitates the interpretation of test results. For dichotomously scored items, earlier research surveyed the theory and methods of an invariant ordering in a nonparametric IRT context. Here the focus is on polytomously scored items, and both nonparametric and parametric IRT models are considered.

The absence of the IIO property in two nonparametric polytomous IRT models is discussed, and two nonparametric models are discussed that imply an IIO. A method is proposed that can be used to investigate whether empirical data imply an IIO. Furthermore, only two parametric polytomous IRT models are found to imply an IIO. These are the rating scale model (Andrich, 1978) and a restricted rating scale version of the graded response model (Muraki, 1990). Well-known models, such as the partial credit model (Masters, 1982) and the graded response model (Samejima, 1969), do no imply an IIO.
Bayesian Item Selection Criteria for Adaptive Testing
Authors: van der Linden Wim J.
Published: February 2006

Owen (1975) proposed an approximate empirical Bayes procedure for item selection in computerized adaptive testing (CAT). The procedure replaces the true posterior by a normal approximation with closed-form expressions for its first two moments. This approximation was necessary to minimize the computational complexity involved in a fully Bayesian approach but is no longer necessary given the computational power currently available for adaptive testing. This paper suggests several item selection criteria for adaptive testing which are all based on the use of the true posterior. Some of the statistical properties of the ability estimator produced by these criteria are discussed and empirically characterized.
Stochastic Order in Dichotomous Item Response Models for Fixed, Adaptive, and Multidimensional Tests
Authors: van der Linden Wim J.
Published: February 2006

Dichotomous IRT models can be viewed as families of stochastically ordered distributions of responses to test items. This paper explores several properties of such distributions. In particular, it is examined under what conditions stochastic order in families of conditional distributions is transferred to their inverse distributions, from two families of related distributions to a third family, or from multivariate conditional distributions to a marginal distribution. The main results are formulated as a series of theorems and corollaries which apply to dichotomous IRT models. One part of the results holds for unidimensional models with fixed item parameters. The other part holds for models with random item parameters as used, for example, in adaptive testing or for tests with multidimensional abilities.
A Case of Extreme Simplicity of the Core Matrix in Three-Mode Principal Components Analysis
Authors: Murakami Takashi , Ten Berge Jos M. F. , Kiers Henk A. L.
Published: February 2006

In three-mode Principal Components Analysis, the P ×Q ×R core matrix G can be transformed to simple structure before it is interpreted. It is well-known that, when P = QR, G can be transformed to the identity matrix, which implies that all elements become equal to values specified a priori. In the present paper it is shown that, when P = QR − 1, G can be transformed to have nearly all elements equal to values specified a priori. A closed-form solution for this transformation is offered. Theoretical and practical implications of this simple structure transformation of G are discussed.
Conditions for Factor (in)Determinacy in Factor Analysis
Authors: Krijnen Wim P. , Dijkstra Theo K. , Gill Richard D.
Published: February 2006

The subject of factor indeterminacy has a vast history in factor analysis (Guttman, 1955; Lederman, 1938; Wilson, 1928). It has lead to strong differences in opinion (Steiger, 1979). The current paper gives necessary and sufficient conditions for observability of factors in terms of the parameter matrices and a finite number of variables. Five conditions are given which rigorously define indeterminacy. It is shown that (un)observable factors are (in)determinate. Specifically, the indeterminacy proof by Guttman is extended to Heywood cases. The results are illustrated by two examples and implications for indeterminacy are discussed.
Covariance Structure Analysis of Ordinal Ipsative Data
Authors: Chan Wai , Bentler Peter M.
Published: February 2006

Data are ipsative if they are subject to a constant-sum constraint for each individual. In the present study, ordinal ipsative data (OID) are defined as the ordinal rankings across a vector of variables. It is assumed that OID are the manifestations of their underlying nonipsative vector y, which are difficult to observe directly. A two-stage estimation procedure is suggested for the analysis of structural equation models with OID. In the first stage, the partition maximum likelihood (PML) method and the generalized least squares (GLS) method are proposed for estimating the means and the covariance matrix of Acy, where Ac is a known contrast matrix. Based on the joint asymptotic distribution of the first stage estimator and an appropriate weight matrix, the generalized least squares method is used to estimate the structural parameters in the second stage. A goodness-of-fit statistic is given for testing the hypothesized covariance structure. Simulation results show that the proposed method works properly when a sufficiently large sample is available.
Mixtures of (Constrained) Ultrametric Trees
Authors: Wedel Michel , DeSarbo Wayne S.
Published: February 2006

This paper presents a new methodology concerned with the estimation of ultrametric trees calibrated on subjects' pairwise proximity judgments of stimuli, capturing subject heterogeneity using a finite mixture formulation. We assume that a number of unobserved classes of subjects exist, each having a different ultrametric tree structure underlying the pairwise proximity judgments. A new likelihood based estimation methodology is presented for those finite mixtures of ultrametric trees, that accommodates ultrametric as well as other external constraints. Various assumptions on the correlation of the error of the dissimilarities are accommodated. The performance of the method to recover known ultrametric tree structures is investigated on synthetic data. An empirical application to published data from Schiffman, Reynolds, and Young (1981) is provided. The ability to deal with external constraints on the tree-topology is demonstrated, and a comparison with an alternative clustering based method is made.
Locally Dependent Latent Trait Model and the Dutch Identity Revisited
Authors: Ip Edward H.
Published: February 2006

In this paper, we propose a class of locally dependent latent trait models for responses to psychological and educational tests. Typically, item response models treat an individual's multiple response to stimuli as conditional independent given the individual's latent trait. In this paper, instead the focus is on models based on a family of conditional distributions, or kernel, that describes joint multiple item responses as a function of student latent trait, not assuming conditional independence. Specifically, we examine a hybrid kernel which comprises a component for one-way item response functions and a component for conditional associations between items given latent traits. The class of models allows the extension of item response theory to cover some new and innovative applications in psychological and educational research. An EM algorithm for marginal maximum likelihood of the hybrid kernel model is proposed. Furthermore, we delineate the relationship of the class of locally dependent models and the log-linear model by revisiting the Dutch identity (Holland, 1990).
On Uniqueness in Candecomp/Parafac
Authors: ten Berge Jos M. F. , Sidiropoulos Nikolaos D.
Published: February 2006

One of the basic issues in the analysis of three-way arrays by CANDECOMP/PARAFAC (CP) has been the question of uniqueness of the decomposition. Kruskal (1977) has proved that uniqueness is guaranteed when the sum of thek-ranks of the three component matrices involved is at least twice the rank of the solution plus 2. Since then, little has been achieved that might further qualify Kruskal's sufficient condition. Attempts to prove that it is also necessary for uniqueness (except for rank 1 or 2) have failed, but counterexamples to necessity have not been detected. The present paper gives a method for generating the class of all solutions (or at least a subset of that class), given a CP solution that satisfies certain conditions. This offers the possibility to examine uniqueness for a great variety of specific CP solutions. It will be shown that Kruskal's condition is necessary and sufficient when the rank of the solution is three, but that uniqueness may hold even if the condition is not satisfied, when the rank is four or higher.
Testing the Significance of the Successive Components in Redundancy Analysis
Authors: Lazraq Aziz , Cléroux Robert
Published: February 2006

In this paper we study the interrelationships between two sets of data measured on the same subjects via redundancy analysis. We consider redundancy analysis from an inferential point of view. Under the hypothesis of multinormality, tests of significance are obtained for each successive redundancy component so that only the significant factors are retained for prediction purposes. An example illustrates the method.
On Robust Estimation of Effect Size Under Semiparametric Models
Authors: Zhang Zhiyi , Schoeps Nancy
Published: February 2006

Estimation of effect size is of interest in many applied fields such as Psychology, Sociology and Education. However there are few nonparametric estimators of effect size proposed in the existing literature, and little is known about the distributional characteristics of these estimators. In this article, two estimators based on the sample quantiles are proposed and studied. The first one is the estimator suggested by Hedges and Olkin (see page 93 of Hedges & Olkin, 1985) for the situation where a treatment effect is evaluated against a control group (Case A). A modified version of the robust estimator by Hedges and Olkin is also proposed for the situation where two parallel treatments are compared (Case B). Large sample distributions of both estimators are derived. Their asymptotic relative efficiencies with respect to the normal maximum likelihood estimators under several common distributions are evaluated. The robust properties of the proposed estimators are discussed with respect to the sample-wise breakdown points proposed by Akritas (1991). Simulation studies are provided in which the performing characteristics of the proposed estimator are compared to that of the nonparametric estimators by Kraemer and Andrews (1982). Interval estimation of the effect sizes is also discussed. In an example, interval estimates for the data set in Kraemer and Andrews (1982) are calculated for both cases A and B.
Fitting Arma Time Series by Structural Equation Models
Authors: van Buuren Stef
Published: February 2006

This paper outlines how the stationary ARMA (p, q) model can be specified as a structural equations model. Maximum likelihood estimates for the parameters in the ARMA model can be obtained by software for fitting structural equation models. For pure AR and mixed ARMA models, these estimates are approximately unbiased, while the efficiency is as good as those of specialized recursive estimators. The reported standard errors are generally found to be valid. Depending on sample size, estimates for pure MA models are biased 5–10% and considerably less efficient. Some assets of the method are that ARMA model parameters can be estimated when only autocovariances are known, that model constraints can be incorporated, and that the fit between observed and modelled covariances can be tested by statistical methods. The method is applied to problems that involve the evaluation of pregnancy as a function of perceived bodily changes, the effect of policy interventions in crime prevention, and the influence of weather conditions on absence from work.
Alternative Solutions for Optimization Problems in Generalizability Theory
Authors: Sanders Piet F.
Published: February 2006

Solutions for the problem of maximizing the generalizability coefficient under a budget constraint are presented. It is shown that the Cauchy-Schwarz inequality can be applied to derive optimal continuous solutions for the number of conditions of each facet.
Testing for DIF in a Model with Single Peaked Item Characteristic Curves: The Parella Model
Authors: Hoijtink Herbert , Molenaar Ivo W.
Published: February 2006

The PARELLA model is a probabilistic parallelogram model that can be used for the measurement of latent attitudes or latent preferences. The data analyzed are the dichotomous responses of persons to items, with a one (zero) indicating agreement (disagreement) with the content of the item. The model provides a unidimensional representation of persons and items. The response probabilities are a function of the distance between person and item: the smaller the distance, the larger the probability that a person will agree with the content of the item. This paper discusses how the approach to differential item functioning presented by Thissen, Steinberg, and Wainer can be implemented for the PARELLA model.
Asymptotic Distributions of the Estimators of Communalities in Factor Analysis
Authors: Ichikawa Masanori
Published: February 2006

Asymptotic distributions of the estimators of communalities are derived for the maximum likelihood method in factor analysis. It is shown that the common practice of equating the asymptotic standard error of the communality estimate to the unique variance estimate is correct for standardized communality but not correct for unstandardized communality. In a Monte Carlo simulation the accuracy of the normal approximation to the distributions of the estimators are assessed when the sample size is 150 or 300.
Full-Information Item Bi-Factor Analysis
Authors: Gibbons Robert D. , Hedeker Donald R.
Published: February 2006

A plausible s-factor solution for many types of psychological and educational tests is one that exhibits a general factor and s − 1 group or method related factors. The bi-factor solution results from the constraint that each item has a nonzero loading on the primary dimension and at most one of the s − 1 group factors. This paper derives a bi-factor item-response model for binary response data. In marginal maximum likelihood estimation of item parameters, the bi-factor restriction leads to a major simplification of likelihood equations and (a) permits analysis of models with large numbers of group factors; (b) permits conditional dependence within identified subsets of items; and (c) provides more parsimonious factor solutions than an unrestricted full-information item factor analysis in some cases.
Accuracy of Kendall's Chi-Square Approximation to Circular Triad Distributions
Authors: Knezek Gerald , Wallace Susan , Dunn-Rankin Peter
Published: February 2006

This paper contains an assessment of the accuracy of Kendall's chi-square approximation to circular triad distributions. Estimated indices are compared to enumerated values across the upper and lower tails of cumulative proportions of circular triad distributions for 5–15 objects. In general, the approximation is found to be quite good. The median difference between the exact distribution and Kendall's approximation is .000692. Critical value approximations are never in error by more than one circular triad and are almost always in the conservative direction. A researcher can be confident that little meaningful error will be present when using Kendall's approximation for experiments outside the range of tabled values. It appears that the chi-square approximation will be accurate to at least two decimal places for distributions beyond 15 objects.
An Item Response Model with Internal Restrictions on Item Difficulty
Authors: Butter René , De Boeck Paul , Verhelst Norman
Published: February 2006

An IRT model based on the Rasch model is proposed for composite tasks, that is, tasks that are decomposed into subtasks of different kinds. There is one subtask for each component that is discerned in the composite tasks. A component is a generic kind of subtask of which the subtasks resulting from the decomposition are specific instantiations with respect to the particular composite tasks under study. The proposed model constrains the difficulties of the composite tasks to be linear combinations of the difficulties of the corresponding subtask items, which are estimated together with the weights used in the linear combinations, one weight for each kind of subtask. Although the model does not belong to the exponential family, its parameters can be estimated using conditional maximum likelihood estimation. The approach is demonstrated with an application to spelling tasks.
Some Remarks on Scheiblechner's Treatment of ISOP Models
Authors: Junker Brian W.
Published: February 2006

Scheiblechner (1995) proposes a probabilistic axiomatization of measurement called ISOP (isotonic ordinal probabilistic models) that replaces Rasch's (1980) specific objectivity assumptions with two interesting ordinal assumptions. Special cases of Scheiblechner's model include standard unidimensional factor analysis models in which the loadings are held constant, and the Rasch model for binary item responses. Closely related are the doubly-monotone item response models of Mokken (1971; see also Mokken & Lewis, 1982; Sijtsma, 1988; Molenaar, 1991; Sijtsma & Junker, 1996; and Sijtsma & Hemker, in press). More generally, strictly unidimensional latent variable models have been considered in some detail by Holland and Rosenbaum (1986), Ellis and van den Wollenberg (1993), and Junker (1990, 1993). The purpose of this note is to provide connections with current research in foundations and nonparametric latent variable and item response modeling that are missing from Scheiblechner's (1995) paper, and to point out important related work by Hemker, Sijtsma, Molenaar, & Junker (1996, 1997), Ellis and Junker (in press) and Junker and Ellis (1997). We also discuss counterexamples to three major theorems in the paper. By carrying out these three tasks, we hope to provide researchers interested in the foundations of measurement and item response modeling the opportunity to give the ISOP approach the careful attention it deserves.
A Family of Chance-Corrected Association Coefficients for Metric Scales
Authors: Zegers Frits E.
Published: February 2006

A chance-corrected version of the family of association coefficients for metric scales proposed by Zegers and ten Berge is presented. It is shown that a matrix with chance-corrected coefficients between a number of variables is Gramian. The members of the chance-corrected family are shown to be partially ordered.
Testing Equality of Correlated Proportions with Incomplete Data on both Responses
Authors: Bhoj Dinesh S. , Snijders Tom A. B.
Published: February 2006

Two test statistics are proposed for testing the equality of two correlated proportions when some observations are missing on both responses. The performance of these tests in terms of size and power is compared with other tests by means of Monte Carlo simulations. The proposed tests are easily computed and compare favorably with other tests.
A Caveat on the use of a Revised Bootstrap Algorithm
Authors: Schluchter Mark D. , Forsythe Alan B.
Published: February 2006

A recently suggested revision of Efron's bootstrap algorithm for small bivariate samples discards bootstrap samples having fewer than 3 distinct points in order to prevent degeneracies in Pearson's r. Examples are given showing that this approach may severely underestimate the variance of sample moments when sample sizes are very small.
Global and Local Covert Visual Attention: Evidence From A Bayesian Hidden Markov Model
Authors: Liechty John , Pieters Rik , Wedel Michel
Published: February 2006

Psychological, psychophysical and physiological research indicates that people switch between two covert attention states, local and global attention, while visually exploring complex scenes. The focus in the local attention state is on specific aspects and details of the scene, and on examining its content with greater visual detail. The focus in the global attention state is on exploring the informative and perceptually salient areas of the scene, and possibly on integrating the information contained therein. The existence of these two visual attention states, their relative prevalence and sequence in time has remained empirically untested to date. To fill this gap, we develop a psychometric model of visual covert attention that extends recent work on hidden Markov models, and we test it using eye-movement data. The model aims to describe the observed time series of saccades typically collected in eye-movement research by assuming a latent Markov process, indicative of the brain switching between global and local covert attention. We allow subjects to be in either state while exploring a stimulus visually, and to switch between them an arbitrary number of times. We relax the no-memory-property of the Markov chain. The model that we develop is estimated with MCMC methodology and calibrated on eye-movement data collected in a study of consumers' attention to print advertisements in magazines.
From Eye Movement to Cognition: Toward a General Framework of Inference Comment on Liechty et al., 2003
Authors: Feng Gary
Published: February 2006

Liechty, Pieters & Wedel (2003) developed a hidden Markov Model (HMM) to identify the states of an attentional process in an advertisement viewing task. This work is significant because it demonstrates the benefits of stochastic modeling and Bayesian estimation in making inferences about cognitive processes based on eye movement data. One limitation of the proposed approach is that attention is conceptualized as an autonomous random process that is affected neither by the overall layout of the stimulus nor by the visual information perceived during the current fixation. An alternative model based on the input-output hidden Markov model (IOHMM; Bengio, 1999) is suggested as an extension of the HMM. The need for further studies that validate the HMM classification results is also discussed.
On the Use of Heterogeneous Thresholds Ordinal Regression Models to Account for Individual Differences in Response Style
Authors: Johnson Timothy R.
Published: February 2006

This paper proposes a general approach to accounting for individual differences in the extreme response style in statistical models for ordered response categories. This approach uses a hierarchical ordinal regression modeling framework with heterogeneous thresholds structures to account for individual differences in the response style. Markov chain Monte Carlo algorithms for Bayesian inference for models with heterogeneous thresholds structures are discussed in detail. A simulation and two examples based on ordinal probit models are given to illustrate the proposed methodology. The simulation and examples also demonstrate that failing to account for individual differences in the extreme response style can have adverse consequences for statistical inferences.
A Hierarchical Bayesian Statistical Framework for Response Time Distributions
Authors: Rouder Jeffrey N. , Sun Dongchu , Speckman Paul L. , Lu Jun , Zhou Duo
Published: February 2006

This paper provides a statistical framework for estimating higher-order characteristics of the response time distribution, such as the scale (variability) and shape. Consideration of these higher order characteristics often provides for more rigorous theory development in cognitive and perceptual psychology (e.g., Luce, 1986). RT distribution for a single participant depends on certain participant characteristics, which in turn can be thought of as arising from a distribution of latent variables. The present work focuses on the three-parameter Weibull distribution, with parameters for shape, scale, and shift (initial value). Bayesian estimation in a hierarchical framework is conceptually straightforward. Parameter estimates, both for participant quantities and population parameters, are obtained through Markov Chain Monte Carlo methods. The methods are illustrated with an application to response time data in an absolute identification task. The behavior of the Bayes estimates are compared to maximum likelihood (ML) estimates through Monte Carlo simulations. For small sample size, there is an occasional tendency for the ML estimates to be unreasonably extreme. In contrast, by borrowing strength across participants, Bayes estimation “shrinks” extreme estimates. The results are that the Bayes estimators are more accurate than the corresponding ML estimators.
The Regression Trunk Approach to Discover Treatment Covariate Interaction
Authors: Dusseldorp Elise , Meulman Jacqueline J.
Published: February 2006

The regression trunk approach (RTA) is an integration of regression trees and multiple linear regression analysis. In this paper RTA is used to discover treatment covariate interactions, in the regression of one continuous variable on a treatment variable withmultiple covariates. The performance of RTA is compared to the classical method of forward stepwise regression. The results of two simulation studies, in which the true interactions are modeled as threshold interactions, show that RTA detects the interactions in a higher number of cases (82.3% in the first simulation study, and 52.3% in the second) than stepwise regression (56.5% and 20.5%). In a real data example the final RTA model has a higher cross-validated variance-accounted-for (29.8%) than the stepwise regression model (12.5%). All of these results indicate that RTA is a promising alternative method for demonstrating differential effectiveness of treatments.