One way to improve this situation is to eliminate the contribution of non-typical (see introduction) words.
the simplest way to archive this is to focus only on saliently occurring words (precisely, words whose occurrences are saliently biased in D(t)) and let the number of words whose saliency is over a threshold value s, denoted by SAL(D(t), s), be the degree of bias of word we need to define the saliency of occurrences of a word and a threshold value with which the occurrences of a word in D(t) is determined as salient.