Hypergeometric language models for republished article finding

Republished article finding is the task of identifying instances of articles that have been published in one source and republished more or less verbatim in another source, which is often a social media source. We address this task as an ad hoc retrieval problem, using the source article as a query. Our approach is based on language modeling. We revisit the assumptions underlying the unigram language model taking into account the fact that in our setup queries are as long as complete news articles. We argue that in this case, the underlying generative assumption of sampling words from a document with replacement, i.e., the multinomial modeling of documents, produces less accurate query likelihood estimates. To make up for this discrepancy, we consider distributions that emerge from sampling without replacement: the central and non-central hypergeometric distributions. We present two retrieval models that build on top of these distributions: a log odds model and a bayesian model where document parameters are estimated using the Dirichlet compound multinomial distribution. We analyse the behavior of our new models using a corpus of news articles and blog posts and find that for the task of republished article finding, where we deal with queries whose length approaches the length of the documents to be retrieved, models based on distributions associated with sampling without replacement outperform traditional models based on multinomial distributions.