paragraph: In practice we don't use raw probability as our metric for evaluating language models, but a variant called perplexity. The perplexity (sometimes called PP for short) perplexity of a language model on a test set is the inverse probability of the test set, normalized by the number of words. For a test set W = w 1 w 2 . . . w N ,:
answer: perplexity of a language model on a test set is the inverse probability of the test set, normalized by the number of words.