When is adjustment of rates not necessary

The same distortion can happen when comparing races, genders, or time periods. Age adjustment can make the different groups more comparable. A "standard" population distribution is used to adjust death and hospitalization rates.

The age-adjusted rates are rates that would have existed if the population under study had the same age distribution as the "standard" population. Therefore, they are summary measures adjusted for differences in age distributions. However, if you compare rates from different sources, it is very important that you use the same standard population on both sides of your comparison.

It is not legitimate to compare adjusted rates which use different standard populations. The rate in the area of study e. This results in an age-specific death rate ASDR per , population for each age group. Each ASDR is then multiplied by the proportion of the standard population see table below that same age group. The age-specific results are summed to get the age-adjusted death rate for the area of study.

The formula is:. This is called the direct method of standardization. A given area's age-specific rate overall or for a given cause is applied to the U. Age-adjusted rates published elsewhere e. The constant or "per population" number used for the age-adjusted rates may vary, depending on the type of event.

For example, the age-adjusted rates for deaths are per , population. However, age-adjusted rates for hospitalizations and procedures are per 10, population and age-adjusted rates for emergency department visits are per 1, population. Age-adjustment is a statistical way to remove confounding caused by age. Age confounding occurs when the two populations being compared have different age distributions and the risk of the disease or outcome varies across the age groups.

The process of age-adjustment by the direct method changes the amount that each age group contributes to the overall rate in each community, so that the overall rates are based on the same age structure. Rates that are based on the same age distribution can be compared to each other without the presence of confounding by age.

Adjustment is accomplished by first multiplying the age-specific rates of disease by age-specific weights. The weights used in the age-adjustment of cancer data are the proportion of the US population within each age group. The weighted rates are then summed across the age groups to give the age-adjusted rate. Age-adjustment is demonstrated here using the cancer mortality rates for all sites of cancer among men in New York State in Age confounding is demonstrated using the prostate cancer mortality rates among white and black men in The crude unadjusted cancer mortality rate is The age-adjusted rate is Based on how many white and black men lived in Upstate in , the crude unadjusted prostate cancer mortality rates were as follows:.

It appears based on this that white men and black men who reside in Upstate New York have the same prostate cancer mortality rate. But this is deceiving because black men in Upstate New York tend to be younger than white men:. In both the older and younger age groups, the prostate cancer mortality rate among black men is over two times higher than among white men.

The age-adjusted rates reflect the higher risk among black men of dying from prostate cancer:.