Likelihood ratios

Likelihood ratios are the ratio of the probability of a specific test result for subjects with the condition, against the probability of the same test result for subjects without the condition.

A likelihood ratio of 1 indicates that the test result is equally likely in subjects with and without the condition. A ratio > 1 indicates that the test result is more likely in subjects with the condition than without the condition, and conversely a ratio < 1 indicates that the test result is more likely in subjects without the condition. The larger the ratio, the more likely the test result is in subjects with the condition than without; likewise the smaller the ratio, the more likely the test result is in subjects without than with the condition.

The likelihood ratio of a positive test result is the ratio of the probability of a positive test result in a subject with the condition (true positive fraction) against the probability of a positive test result in a subject without the condition (false positive fraction). The likelihood ratio of a negative test result is the ratio of the probability of a negative test result in a subject with the condition (false negative fraction) to the probability of a negative test result in a subject without the condition (true negative fraction).