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; assigning a respective confidence measure to each attribute value of said 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, and wherein the correlation-function application step is applied iteratively, said correlation functions are applied only to attribute-values estimates associated with a confidence measure whose value exceeds a threshold, and said confidence measure is set to a maximum value for attribute values provided by the user as the initial estimate to ensure that said attribute values provided by the user as the initial estimate are not changed.