1. A computer-implemented method comprising: a processor determining a probability distribution for a word in a plurality of reference summaries of a document as θ R , i = μ ⁢ ⁢ p ⁡ ( w i | c ) + ∑ L ⁢ a i , j j = 1 μ + ∑ j = 1 L ⁢ a o , j , where θ R,i is the probability distribution of word w i in the plurality of reference summaries, L is a number of reference summaries in the plurality of reference summaries, a i,j is a count of the number of times word w i appears in summary j, p(w i |c) is a probability of word w i given a corpus of documents c, μ is a scaling factor greater than one, and a 0,j is a count of the number of words in summary j; the processor determining a probability distribution for the word in a machine-generated summary as θ A , i = a i + μ ⁢ ⁢ p ⁡ ( w i | c ) a o + μ where θ A,i is the probability distribution of word w i in the machine-generated summary, a i is a count of the number of times word w i appears in the machine-generated summary, p(w i |c) is the probability of word w i given a corpus of documents c, μ is the scaling factor, and a 0 is a count of the number of words in the machine-generated summary; the processor comparing the probability distribution for the word in the reference summaries to the probability distribution for the word in the machine-generated summary to generate a score for a machine-generated summary of the document.