long period of time, which maximizes comparability of published statistics, and revising weights to be close to the current age distribution. When comparing data from a specific country or region, using a standard population from that country or region means that the age-adjusted rates are similar to the true population rates. On the other hand, standardizing data using a widely used standard such as the WHO standard population allows for easier comparison with published statistics. When is it used Age adjustment is commonly used when comparing prevalences in different populations. It is not used to derive life expectancy, which is calculated directly from the age-specific mortality rates, with no reference population required. Age adjustment is also not appropriate when attempting to compare population totals (for instance, if we wanted to know the total number of hospital beds required for patients with circulatory diseases). See also References Further reading Epidemiology Design of experiments Demography