Standardized mortality ratio (indirect age adjustment) tells how many persons, per thousand of the population, will die in a given year and what the causes of death will be. Such statistics have many uses.
Mortality tables are also often used when numbers of deaths for each age-specific stratum are not available. It is also used to study mortaliy rate in an occupationally exposed population: Do people who work in a certain industry, such as mining or construction, have a higher mortality than people of the same age in the general population? Is an additional risk associated with that occupation? To answer the question of whether a population of miners has a higher mortality than we would expect in a similar population that is not engaged in mining, the age-specific rates for such a known population, such as all men of the same age, are applied to each age group in the population of interest. This will yield the number of deaths expected in each age group in the population of interest, if this population had had the mortality experience of the known population. Thus, for each age group, the number of deaths expected is calculated, and these numbers are totaled. The numbers of deaths that were actually observed in that population are also calculated and totaled. The ratio of the total number of deaths actually observed to the total number of deaths expected, if the population of interest had had the mortality experience of the known population, is then calculated. This ratio is called the standardized mortality ratio (SMR). The SMR is defined as follows: SMR = (Observed no. of deaths per year)/(Expected no. of deaths per year).
Gordis, Leon (December 6, 2004). Epidemiology. Saunders; 3 edition. pp. 64–65. ISBN 1416025308.