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Title: Scaling Hierarchical Coreference with Homomorphic Compression

Paper content:
Locality sensitive hashing schemes such as \simhash provide compact representations of multisets from which similarity can be estimated.
However, in certain applications, we need to estimate the similarity of dynamically changing sets.
In this case, we need the representation to be a homomorphism so that the hash of unions and differences of sets can be computed directly from the hashes of operands.
We propose two representations that have this property for cosine similarity (an extension of \simhash and angle-preserving random projections), and make substantial progress on a third representation for Jaccard similarity (an extension of \minhash).
We employ these hashes to compress the sufficient statistics of a conditional random field (CRF) coreference model and study how this compression affects our ability to compute similarities as entities are split and merged during inference.
\cut{We study these hashes in a conditional random field (CRF) hierarchical coreference model in order to compute the similarity of entities as they are merged and split during inference.}
We also provide novel statistical analysis of \simhash to help justify it as an estimator inside a CRF, showing that the bias and variance reduce quickly with the number of bits.
On a problem of author coreference, we find that our \simhash scheme allows scaling the hierarchical coreference algorithm by an order of magnitude without degrading its statistical performance or the model's coreference accuracy, as long as we employ at least 128 or 256 bits.
Angle-preserving random projections further improve the coreference quality, potentially allowing even fewer dimensions to be used.
Probabilistic models in machine learning, such as conditional random fields (CRFs), are widely successful at modeling many problems at the heart of knowledge base construction, including those in natural language processing, information extraction and data integration.
However, when dealing with natural language data, the underlying feature representations are often sparse, high-dimensional and dynamic (i.e., they change during inference).
In this paper we consider the task of coreference resolution, in which the goal is to partition a set of mentions into the entities to which they refer.
We might represent each mention with a feature vector in which each dimension corresponds to a word or n-gram.
Since only a small subset of the vocabulary is observed per mention, most elements of the vector are zero.Given the model and these representations, inference entails making decisions about whether two entities should be coreferent.
To make such decisions, the model estimates a probability that involves computing the similarities between the aggregate feature representations of the two entities' mentions.
Since all the vectors are both sparse and high-dimensional, these similarity operations are computationally expensive because the sparse data structures supplant the dense arrays that would otherwise support fast look-ups.
Moreover, as the inference algorithm makes decisions about whether or not two entities are coreferent, we may have to split or merge the entities and thus we must update the feature vector to reflect these changes.
Maintaining such sparse-vector representations in the inner-loop of probabilistic inference is expensive, especially as the entities grow in size.In order to cope with the computational problems associated with sparse, high dimensional dynamic feature representations, we propose using homomorphic compression, in which the compressed representations of intermediate inference results can be computed directly from their operands, allowing inference to run directly on the compressed representations of the data even as they change.
In this paper, we consider several such schemes to scale hierarchical coreference.
First, we propose a novel homomorphic cosine-preserving hashing scheme based on simhash BID7 that also supports addition and subtraction to more efficiently represent the data and the evolving intermediate results of probabilistic inference.
Second, because various linear norm-preserving random projections also preserve angles BID24 , we can directly compute cosine similarity on projected data -linearity of the projections means that they too can be updated dynamically.
The resulting angle estimates are superior to the homomorphic simhash representation, at the cost of reduced efficiency for certain operations.
Third, we develop a homomorphic version of minhash BID5 to support Jaccard similarity.
Our current algorithm is biased, but the bias appears small in practice for the situations we have considered.
Although the minhash based set representations are not currently employed in hierarchical coreference, they might be useful in other models or applications that involve binary features over changing sets BID9 .We
provide error analysis for all three schemes, collating and extending known results, and in the case of simhash, we provide novel statistical analysis for its use as a direct estimator for cos(θ) that shows the bias and variance decrease rapidly with the number of bits, helping to justify its use in a CRF for a task like coreference. On
a hierarchical model for coreference resolution, the proposed simhash scheme improves the speed of probabilistic inference by an order of magnitude while having little effect on model quality. Moreover
, we find that the underlying random projection representation can provide even better cosine estimates than simhash, at the cost of not being able to use certain fast bitwise-operations to compute similarities. Finally
, we briefly evaluate homomorphic minhash and show that even though there are possible pathological cases, as long as we employ enough hash functions, the estimates are reasonably close to the true Jaccard, albeit, biased.
In this paper we presented several homomorphic compression schemes for representing the sparse, high dimensional features in a graphical model for coreference resolution, even as these features change during inference.
Our primary concern was cosine similarity for which we investigated simhash and angle-preserving random projections.
We also proposed a homomorphic version of minhash for Jaccard similarity.
In the particular case of simhash, we presented a new variant of the original estimator and analyzed its statistical propertiesincluding variance, bias and consistency -to help justify its use in a probabilistic model.
We found that both simhash and angle-preserving random projections were sufficiently accurate, given enough dimensions, to represent features for coreference, and that the random projections produce slightly better estimates.
Moreover, these representations were an order of magnitude faster than conventional sparse data structures, laying the foundation for greater scalability.

assistant: text: We employ linear homomorphic compression schemes to represent the sufficient statistics of a conditional random field model of coreference and this allows us to scale inference and improve speed by an order of magnitude.