likelihood ratio

n. 1. In statistics, the ratio of the likelihood that some fact is true if a condition is true divided by the probability of the act's being true if the condition is not present. Thus one might calculate the ratio between the probability of a person's having a cough if the person has flu and divide it by the probability of a person's having a cough who does not have flu. 2. In signal detection theory, it is the ratio of the likelihood that a response is due to a signal plus background noise divided by the probability that a response is due only to background noise at a particular cutoff criterion level.