17. The computer-implemented method of claim 16 , wherein the training comprises: processing each of the reference summaries to determine for each reference summary a first numerical measure indicative of a similarity between the summary and a particular reference summary, the particular reference summary having been designated as representative of the subject matter of the text, a second numerical measure indicative of a degree to which a single sentence of the reference summary summarizes an entirety of the text, and a third numerical measure indicative of a degree of copying in the reference summary of multi-word sequences present in the text; and conducting a numerical regression analysis based on the first, second and third numerical measures and reference score for each of the plurality of reference summaries to determine the first, second and third weighting factors.
17. The computer-implemented method of claim 16 , wherein the training comprises: processing each of the reference summaries to determine for each reference summary a first numerical measure indicative of a similarity between the summary and a particular reference summary, the particular reference summary having been designated as representative of the subject matter of the text, a second numerical measure indicative of a degree to which a single sentence of the reference summary summarizes an entirety of the text, and a third numerical measure indicative of a degree of copying in the reference summary of multi-word sequences present in the text; and conducting a numerical regression analysis based on the first, second and third numerical measures and reference score for each of the plurality of reference summaries to determine the first, second and third weighting factors. 18. The computer-implemented method of claim 17 , wherein the determining of the third numerical measure for each reference summary includes: determining a first metric for the reference summary, the first metric being a ratio between a first value and a second value, wherein the first value is a sum of lengths of all three-word or longer phrases from the text that are included in the reference summary, and wherein the second value is a length of the reference summary; determining a second metric for the reference summary, the second metric being a ratio between the first value and a third value, wherein the third value is a length of the text; and determining a third metric for the reference summary, the third metric being a length of a longest word sequence from the text that is included in the reference summary.