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Example of Standardization of State Prevalence to National Race/Ethnic Distributions

Background

The prevalence of overweight for children birth to 5 years of age is 13.2% for a state WIC program and this is similar to the national PedNSS prevalence of 12.8%. However, the race/ethnic distribution for the state WIC program is considerably different from the national race/ethnic distribution. For example, the national PedNSS has a higher proportion of Hispanic children, 31.7% compared to 12.6% in the state WIC program.

In this example, the state WIC program wants to know how their prevalence of overweight would change if they had the same race/ethnic distribution as the national PedNSS data.

Standardization is a technique used to make a population comparable to another population on a specific demographic characteristic thus, making the prevalence of the health indicator of interest also comparable. In this example, standardization allows you to determine what the prevalence of overweight for children in the state WIC program would be based on the race/ethnic distribution of the national PedNSS population. Therefore, the state prevalence of overweight will be standardized based on the race/ethnic distribution of the nation using the following formula.

Standardization Formula

Σ (A × B) = Standardized Rate or Adjusted Prevalence

Σ is the sum of A (proportion of the characteristic in the standard population) multiplied by B (population characteristic specific prevalence of health indicator) and equals the standardized rate or adjusted prevalence.

Calculation

The following table will be used to calculate the state standardized or adjusted prevalence of overweight based on the race/ethnic distribution of the nation.

Step 1

From PedNSS Table 1D, Summary of Demographic Indicators, obtain the national distribution of race/ethnicity and list the percent as decimal values defined as “A” in the table.

Sample: PedNSS Table 1D, Summary of Demographic Indicators

Step 2

From PedNSS Table 8C, Growth and Anemia Indicators By Race/Ethnicity or Age, obtain the race/ethnic specific prevalence of overweight for the state WIC program and list the prevalence defined as “B” in the table.

Sample: PedNSS Table 8C, Growth and Anemia Indicators
By Race/Ethnicity or Age

Step 3

Multiply the proportion for each race/ethnic group for the nation (A) by the state overweight prevalence for each corresponding race/ethnic group (B); the sum of the product of A × B for each race/ethnic group is the adjusted prevalence for overweight for the state. The table below shows this calculation for each race/ethnic group.

Summary

The adjusted prevalence for overweight for the state WIC program is the prevalence if the race/ethnic distribution in the state WIC program had been identical to that of the national PedNSS. The state race/ethnic adjusted prevalence of overweight is 13.9%, higher than the state’s original prevalence of 13.2% and the national PedNSS prevalence of 12.8%.

If we had not standardized the state prevalence we would have concluded that the state prevalence of 13.2% was slightly higher (0.4%) than the national prevalence (12.8%), when in fact, the race/ethnic adjusted prevalence of 13.9% is 1% higher than that of the nation.

Therefore, if the state actually had the same race/ethnic distribution as the nation, the adjusted prevalence of overweight (13.9%) in the state would be higher than the original state prevalence (13.2%) indicates.

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This page last updated April 11, 2005

United States Department of Health and Human Services
Centers for Disease Control and Prevention
National Center for Chronic Disease Prevention and Health Promotion
Division of Nutrition and Physical Activity