1. A method of producing a set of tags for an input audiovisual file, the set of tags indicating values of attributes of an audiovisual work of defined type represented by said audiovisual file, the method comprising the steps of: issuing, to a user, a prompt for manual assignment of tags to said input audiovisual file; inputting, as an initial estimate of the values of the respective attributes of the audiovisual work represented by said audiovisual file, data representative of the tags assigned to said input audiovisual file by the user in response to said prompt; automatically applying a set of one or more correlation functions to the attribute-value estimates of said initial estimate, to produce a set of revised estimates; and outputting the final result of the applying step as the set of tags for said input audiovisual file; wherein the correlation functions applied in said applying step are functions embodying the correlations holding between known attribute-values of a set of training examples, said training examples being audiovisual works of said defined type corresponding to manually-tagged audiovisual files.