Editor,—Carpenter has written an important study on prevalence and distribution of bruising in babies.1 However, there are problems with the terms relating to measures of disease frequency in epidemiology which, arguably, make this descriptive paper misleading.

The terms point prevalence,prevalence proportion,prevalence ratio, andprevalence rate are sometimes used to mean the same thing.2 However, the wordrate is used more specifically in the context of the number of events per unit time and intuitively suggests a survival rate or an incidence rate. This is strongly purported by the use of the term rate parameter to describe the unknown and estimated value associated with a Poisson probability model.3 The distinction is often not explicit, even in the best textbooks of epidemiology, but I think that it is worth making.

Carpenter's study included infants aged between 6 and 12 months who were …