1. A method of information retrieval, comprising: determining, using a microprocessor, Q generative models (λ) in accordance with Probabilistic Latent Semantic Indexing (PLSI), said Q generative models being determined in an offline training; receiving a user query (q); choosing, using said microprocessor, N generative models out of the Q generative models, with N<Q; determining, using said microprocessor, a content item (d) based on said query and a combination of the N generative models.