Take for instance equation 67 and 68 from this chapter:the value of  $P(q|R=1,q)$ can become zero if the term is not present in the document, andas all probabilities are multiplied, the probability over the whole document will become zero.It seems that $P(d|R=1,q)$ will always come to zero as soon as some of the index terms are missing in the document?
Why is the zero value ignored while deriving a ranking function for query terms in Probabilistic IR?
pr.probability;ir.information retrieval