1. A method performed by a computer having a processor and a memory for generating a document summary, the method comprising: with the processor, providing a first document model for a document, the first document model is a statistical language model with a probability distribution that reflects a frequency with which trigrams occur in the document and which is based only on the document itself, the probability distribution is generated by: counting a number of occurrences of each trigram within the document, for each bigram, determining a number of occurrences of the bigram within the document by summing the number of occurrences of each trigram within the document that begins with the bigram, and for each trigram, calculating a probability of observing the trigram in the document by dividing the number of occurrences of the trigram within the document by the number of occurrences of the bigram within the document that comprises a first two words of the trigram and storing the calculated probability in the memory; calculating scores based on normalized probabilities for sentences of the document based on the first document model, a normalized probability is a product of the probabilities of the trigrams in a sentence adjusted by a length of the sentence; selecting one or more sentences based on the scores of the sentences to form the summary of the document; storing the summary in the memory; generating a second document model for the document and the generated summary that indicates probabilities by which the various trigrams occur in the document and the generated summary; for each portion of the summary, calculating a score based on the probabilities indicated by the second document model; and combining the calculated score of each portion of the summary to generate an overall score for assessing the effectiveness of the summary.