Numerator-Denominator Bias

Synonyms

Numerator-denominator mismatch

Definition

A numerator-denominator bias is a systematic distortion due to a denominator that does not match the numerator, or vice versa. For example, when calculating the mortality rate of a population, a numerator-denominator bias will occur if the numerator does not relate to the population in the denominator. This would be the case if the numerator contained all deaths that occurred within a particular country, whereas the denominator contained only the resident population. The mortality rate calculated on this basis would systematically overestimate the true mortality rate of the resident population as deaths among visiting foreigners would be included in the numerator, but the foreign population is not part of the denominator.