11. The method of claim 5 , wherein the difference score is determined as an inverse of a similarity score, where the similarity score is defined as similarity ⁡ ( A , B ) =  S ⁡ ( A ) ⋂ S ⁡ ( B )   S ⁡ ( A ) ⋃ S ⁡ ( B )  , where A is the particular version of the document and B is the reference version of the document.
5. The method of claim 1 , wherein determining the weighted overall quality of result statistic comprises: determining a respective difference score for each of the plurality of versions of the document with reference to the reference version of the document, wherein the difference score for a particular version in the plurality of versions of the document and the reference version of the document measures a difference between a representation of the particular version and a representation of the reference version of the document; and weighting each version-specific quality of result statistic by a weight derived from the difference score for the version of the document associated with the version-specific quality of result statistic.