10. A system, comprising: a data processing apparatus in data communication with resource index data store, the resource index data indexing data of the resources and resource locators addressing the resources; a data storage apparatus in data communication with the data processing apparatus and storing instructions executable by the data processing apparatus and upon execution by the data processing apparatus cause the data processing apparatus to perform operations comprising: selecting, by the data processing apparatus, object representations from a dataset storing a plurality of object representations, each object representation being an association of: an object identifier that identifies an object instance in a dataset and corresponds to an object; a context value that identifies a context of the object; and a set of feature values that identify features of the object; wherein each object identifier is unique in the dataset, and each context value is associated with one or more object identifiers; for each feature value: determining an inter-context score that is proportional to the number of different context values in the dataset that are associated with the feature value, wherein the inter-context score is a probability that a random pair of object representations that each include a particular feature value are each associated with different context values and is determined independent of inter-context scores of other feature values; and determining an intra-context score that is proportional to the number of times the feature value is associated with each context value, wherein the intra-context score is a probability that a random pair of object representations that each include the particular feature value are each associated with the same context value and is determined independent of intra-context scores of other feature values; and for a selected pair of object representations, determining a similarity score based on inter-context scores and intra-context scores of matching feature values in the set of features for the pair of object representations, the similarity score being a measure of the similarity of the object representations in the pair of object representations, the determining comprising generating a respective set of feature weights for the set of features of the selected pair of object representations, each feature weight in the respective set of feature weights corresponding to a respective feature and being determined from the inter-context score and intra-context score for the respective feature and further determined independent of the inter-context score and intra-context score for other respective features.
10. A system, comprising: a data processing apparatus in data communication with resource index data store, the resource index data indexing data of the resources and resource locators addressing the resources; a data storage apparatus in data communication with the data processing apparatus and storing instructions executable by the data processing apparatus and upon execution by the data processing apparatus cause the data processing apparatus to perform operations comprising: selecting, by the data processing apparatus, object representations from a dataset storing a plurality of object representations, each object representation being an association of: an object identifier that identifies an object instance in a dataset and corresponds to an object; a context value that identifies a context of the object; and a set of feature values that identify features of the object; wherein each object identifier is unique in the dataset, and each context value is associated with one or more object identifiers; for each feature value: determining an inter-context score that is proportional to the number of different context values in the dataset that are associated with the feature value, wherein the inter-context score is a probability that a random pair of object representations that each include a particular feature value are each associated with different context values and is determined independent of inter-context scores of other feature values; and determining an intra-context score that is proportional to the number of times the feature value is associated with each context value, wherein the intra-context score is a probability that a random pair of object representations that each include the particular feature value are each associated with the same context value and is determined independent of intra-context scores of other feature values; and for a selected pair of object representations, determining a similarity score based on inter-context scores and intra-context scores of matching feature values in the set of features for the pair of object representations, the similarity score being a measure of the similarity of the object representations in the pair of object representations, the determining comprising generating a respective set of feature weights for the set of features of the selected pair of object representations, each feature weight in the respective set of feature weights corresponding to a respective feature and being determined from the inter-context score and intra-context score for the respective feature and further determined independent of the inter-context score and intra-context score for other respective features. 11. The system of claim 10 , wherein determining a similarity score comprises determining, for each feature value, a feature weight based on a ratio of the inter-context score of the feature value to the intra-context score of the feature value.