In direct age-adjustment, a common age-structured population is used as standard. This population may actually exist (e.g., United States population, 1999) or may be fictitious (e.g., two populations may be combined to create a standard). In indirect age-adjustment, a common set of age-specific rates is applied to the populations whose rates are to be standardized. The simplest and most useful form of indirect adjustment is the standardized mortality ratio (SMR) (5).
Keywords: easy way, standardization methods, direct, indirectComparing mortality and morbidity rates in two or more different geographic areas is important for the evaluation of community health status. As there is a possibility of having different frequency distributions in different populations, a comparison between crude rates would be misleading since crude rates are not very informative about the health status of a population. Standardization for the characteristic(s) responsible for the differences in comparison is necessary. Age and sex are two of the most common variables used for standardization and they are called standardized rates. The difference between crude rates and standardized rates is that crude rates are calculated based on the population under study as a whole whereas standardized rates are based on particular characteristic(s) as standard ( Figure 1 ). If the rates are calculated based on the specific characteristic(s), they are called specific rates (e.g. age specific mortality rate).
Concept of direct standardization
This article attempts to help health personnel in the selection and utilization of appropriate standardization methods using illustrated explanations. There are two methods for calculating standardized rates, namely direct and indirect standardization. For the example purpose, let us concentrate on the standardization methods based on age-standardized rates.
When age-specific mortality rates for two or more populations are known, direct standardization method can be applied.
Calculate the age-specific mortality rates for each age group in each population. Then choose the standard (reference) population from one of the populations (*Note: If the mortality rates of a specific community are compared to the national population, then the national population is considered as a “standard” population). Multiply the age-specific mortality rates of the other population under study to the number of persons in each age group of the standard population. By this way, you will get the expected deaths for each age group of each population. Add the number of expected deaths from all age groups. Finally to get the age-adjusted mortality rates, divide the total number of expected deaths by the standard population (1–4). Now you can conclude by comparing the age-standardized mortality rates of two populations ( figure 2 ).
Procedure for application of direct standardization method
