Epidemiology – Standardization – Flashcards

Crude mortality rates can be misleading. We can see more clearly why this happens if we divide up our data by age-group - in other words, if we stratify the data by age-group.

A special application of stratification Used to control for the confounding effects of age so that rates of disease or mortality can be compared in populations with different age structures. There are two methods of standardization: Direct Indirect.

Use observed age-specific mortality rates from each population Apply them to a specified standard population with a known age structure. This gives an age-adjusted number of deaths for each study population which can be used for comparisons.

Find the total number of deaths that would have been expected if Sweden had had the same age structure as the standard population. = 570.8 per 1000 Divide above by the total number of person-years in the standard population. = 100 Result is an age-adjusted (or age-standardized) mortality rate for Sweden. 570.8/100=5.71 per 1000 person yrs.

Calculate age standardized mortality rate for Ecuador. Calculate an age-standardized rate ratio by dividing the age-adjusted rate for Ecuador by that for Sweden. This measure is called the comparative mortality figure (CMF)

Sweden Age-adjusted mortality rate = 5.7 per 1000 pyrs Ecuador Age-adjusted mortality rate = 7.8 per 1000 pyrs CMF = 7.8 / 5.7 = 1.37

The standard population used can be any population Could even be one of the populations we are studying. It is important to specify which standard population was used The choice of standard population can affect the results of the comparison.

To overcome the problem of the dependence of the CMF on the choice of standard population, "ideal" standard populations have been created: Examples The African standard The European standard The World standard

Another approach to comparing mortality in populations. Used when numbers of deaths for each age-specific stratum are not available Also used to study mortality in an occupationally exposed population

For example: 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? To answer this question: Apply age-specific death rates from the general population (standard) to the real age structure of the study population (miners or construction workers) to determine the expected deaths. Then compare the total number of expected (calculated) deaths with the number actually observed.

The ratio of observed to expected deaths calculated in this way is called a standardized mortality ratio (SMR) Usually multiplied by 100 and expressed as a percentage An SMR of 150%. Means that 50% more deaths are observed than we would expect if the study population (miners) had the same mortality pattern as the standard population.

Direct Standardization Requires that age specific rates of mortality in population The age structure of the study population is known Indirect Standardization Requires that the total number of deaths and the age structure of the study population Preferable when there are small numbers in age groups