Much of the birth and death information published by governments is in absolute numbers. These raw data are difficult to interpret. For example, a comparison of the 42,087 births in Utah with the 189,392 in Florida in 1996 reveals nothing about the relative levels of fertility because Florida has a larger population (Ventura et al. 1998, p. 42).

To control for the effect of population size, analyses of fertility and mortality usually use rates. A rate measures the number of times an event such as birth occurs in a given period of time divided by the population at risk to that event. The period is usually a year, and the rate is usually expressed per 1,000 people in the population to eliminate the decimal point. Dividing Florida's births by the state's population and multiplying by 1,000 yields a birth rate of 13 per 1,000. A similar calculation for Utah yields 21 per...