The performance measure for infection control, such as the number of primary bloodstream infections per 1000 central line days, is an example of a rate. In epidemiology and public health, a rate is a measure of the frequency with which an event, such as a new case of illness, occurs in a population over a period of time. The denominator is the population at risk; the numerator is the number of occurrences of disease. Here, the number of primary bloodstream infections is the numerator, and the number of central line days is the denominator. Therefore, this measure is a rate.