Adjusted Rates Nancy D. Barker

Adjusted Rates

Crude Rates Table 1

Adjusted Rates Study question: 1. What do you think? Is it far more hazardous to live in Arizona than Alaska?

Adjusted Rates Study question: 1. What do you think? Is it far more hazardous to live in Arizona than Alaska? These two rates are crude rates because they represent the overall mortality experience in 1996 for the entire population of each state. Crude rates do not account for any differences in these populations on factors such as age, race or sex that might have some influence on mortality. Without consideration of such factors, it would be premature to conclude that it is far more hazardous to live in Arizona than Alaska.

Population Distribution by Age Study Questions: 2. Which population is older? 3. Why should we expect relatively more deaths in Arizona than in Alaska?

Population Distribution by Age Study Questions: 2. Which population is older? Arizona. The dry warm climate of Arizona attracts many older persons than does Alaska. 3. Why should we expect relatively more deaths in Arizona than in Alaska? There are relatively more older persons living in Arizona, and older persons are at high risk of dying.

Population Distribution by age Table 2

Adjusted Rates

Age-Adjusted Rates

Age-Specific Death Rates Table 3 * Rate per 100,000

Direct Method

Averaging specific rates (e.g., age-specific) for the study population using the distribution of a select standard population as weights. Needed for Direct adjustment: 1.Specific rates for the study population 2.Distribution for the selected standard population across the same strata as those used in determining specific rates.

Direct Method * Rate per 100,000 ** Expected death rates if the two states had the same population distribution as the US in 1996

Direct Method Alaska: Expected number of deaths over all age groups combined: 2,271, US population: 265,405,721 Adjusted Rate: (2,271,873.6/265,405,721*100,000) = 856.0

Direct Method Arizona: Expected number of deaths over all age groups combined: 2.208, US population: 265,405,721 Adjusted Rate: (2,208,441.6/265,405,721*100,000) = 832.1

Crude Vs. Adjusted Rates Study Questions: 1.How do we interpret these new age-adjusted results? 2.Bases on these results, how do you think the age distribution of Alaska compares to that of the 1996 US population? 3.How do you think the age distribution of Arizona compares to that of the 1996 US population?

Crude Vs. Adjusted Rates Study Questions: 1.How do we interpret these new age-adjusted results? Controlling for any age differences in the two populations, the overall mortality rate is higher in Alaska with a cold, damp climate, then in Arizona where the climate is warm and dry. 2.Bases on these results, how do you think the age distribution of Alaska compares to that of the 1996 US population? The population of Alaska must be much younger than the US population since the age-adjusted rate was so much higher than the crude rate. 3.How do you think the age distribution of Arizona compares to that of the 1996 US population? The rate for Arizona did not change much from crude to adjusted because Arizona’s distribution was only slightly older that that of the entire US in 1996

Adjusted Rates Used to compare rates across populations that are different with respect to factors that are associated with the health event being investigated.

Choice of the Standard Population Representative of the study population being compared. Understand how choice affects directly standardized rates.

Choice of Standard Population Candidates: 1.Combined or pooled population of the population being studied. 2.Population of one of the study groups. 3.A large population. 4.A hypothetical population

Indirect Method Averaging specific rates for a select standard population using the distribution of the study population as weights. (use when any of the specific rates in the study population are unavailable.) Needed for indirect adjustment: 1.Specific rates for the selected standard population. 2.Distribution for the study population across the same strata as those used in calculating the specific rates in the standard population. 3.Crude rate for the study population. 4.Crude rate for the standard population.

Why Adjust? Standardization adjusts for confounding variables. A summary measure for a population is easier to compare with similar summary measures than are sets of specific rates. Specific rates may be imprecise or unstable because of sparse data.

Caution No meaning should be attached to the magnitudes of adjusted rates, only to comparisons between adjusted rates for two or more populations.