Crucial to rate calculation is the determination of an appropriate denominator. For many mortality rates, such as the crude death rate and age- and cause-specific death rates, and also some incidence rates, denominators typically derive from the census. However, population census data perform another very useful function in descriptive epidemiology. They furnish the building blocks for constructing population pyramids. Valuable for comparisons, population pyramids graphically display the composition of a population broken down by age and sex. Although a pyramid is a cross-sectional snapshot of a population, it also reflects that population’s history.