8. The system of claim 7 , wherein the one or more metadata attributes comprise a plurality of metadata attributes, wherein the act (B) comprises: automatically evaluating multiple of the plurality of metadata attributes, the automatically evaluating a first of the multiple metadata attributes comprising generating at least one language model for at least one group of training data instances obtained by dividing the training data instances based on their respective values for the first metadata attribute; and identifying the set of metadata attributes based on results of the evaluation.
8. The system of claim 7 , wherein the one or more metadata attributes comprise a plurality of metadata attributes, wherein the act (B) comprises: automatically evaluating multiple of the plurality of metadata attributes, the automatically evaluating a first of the multiple metadata attributes comprising generating at least one language model for at least one group of training data instances obtained by dividing the training data instances based on their respective values for the first metadata attribute; and identifying the set of metadata attributes based on results of the evaluation. 9. The system of claim 8 , wherein automatically evaluating the first metadata attribute comprises: dividing the training data instances, based on their respective values for the first metadata attribute, to obtain two or more groups of training data instances; generating a child language model for each of the obtained groups of training data instances; generating a parent language model using all the training data instances; and calculating a score for the first metadata attribute based at least in part how a measure of the goodness of fit of the generated child language models compares with a measure of the goodness of fit of the generated parent language model.