1. A method, implemented by one or more computing devices, for generating a model, comprising: sampling user-behavioral data from a repository of user-behavioral data, the user-behavioral data identifying linguistic items submitted by users, together with selections made by the users in response to the linguistic items; sampling knowledge data from one or more structured knowledge resources, the knowledge data representing relationships among linguistic items expressed by said one or more structured knowledge resources; and generating a model on the basis of the user-behavioral data and the knowledge data, using a machine-learning training process, the model providing logic for assessing relevance of linguistic items, said sampling of user-behavioral data, said sampling of knowledge data, and said generating of the model being performed using at least one processing device associated with said one or more computing devices.