Mental Disorder: Social Epidemiology
Computing ratios is a primary method by which epidemiologists describe the extent of health problems in a particular population. The admissions rate for state and county mental hospitals as discussed earlier is an example of a crude rate. A crude rate is the simplest ratio calculated by epidemiologists; it deals with just the number of persons (cases) who have the characteristics being measured during a specific unit of time. Birth and death rates are other examples of crude rates. Crude rates, however, may be too gross a measure to be useful in understanding significant differences within the population; therefore,
other rates are used to measure specific variables such as age, sex, race, or any other characteristics of interest. Age-specific rates are computed in the same way as crude rates, except that the numerator and the denominator are confined to a specific age group (a similar method can be used to determine sex-specific rates, race-specific rates, and the like). To calculate an age-specific rate, the procedure is to subdivide a population by age and then compare the number of cases in that sub-population with the total number of persons within the subpopulation. If one wanted to compute the 2016 age-specific state and county mental hospital admissions rates for all U.S. 18-24-year-olds, one would need to know how many 18-24-year-olds there were in that year and the number of admissions there were in that age-specific group.