How To... - Interpret Data - When
Example of Standardization of Race/Ethnicity for a Population Over Time

Background

The prevalence of obesity for children birth to 5 years of age increased in the state WIC program from 9.4% in 1992 to 11.8% in 2001. During this time period the race/ethnic distribution in the state WIC program changed markedly. From 1992 to 2001 the proportion of white children decreased from 43.1% to 35.2%, and the proportion of black children decreased from 38.3% to 34.1%, while the proportion of Hispanic children increased from 17.7% to 29.4%.

In this example, the state WIC program wants to determine if the increase in the prevalence of overweight from 1992 to 2001 is because of the change in the distribution of the population especially the increase in the proportion of Hispanic children who also have the highest prevalence of obesity. The race/ethnic distribution of the most current or last year of trend data will be used as the standard because it is most relevant.

Standardization is a technique used to make the population more comparable on a specific demographic characteristic thus, making the prevalence of the health indicator of interest also comparable. It will allow you to determine what the prevalence of obesity for children in 1992 would have been based on the race/ethnic distribution of the population in 2001. Therefore, the state prevalence of obesity in 1992 will be standardized or adjusted based on the race/ethnic distribution of the state in 2001 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 1992 prevalence based on the race/ethnic distribution of the state in 2001.

Step 1

From the 2001 PedNSS Table 10C, Summary of Trends in Racial and Ethnic Distribution, obtain the state WIC program distribution of race/ethnicity in 2001 and list the percent as decimal values defined as "A" in the table.

Sample: PedNSS Table 10C, Summary of Trends in Growth and Anemia Indicators

Step 2

From the 2001 PedNSS Table 18C, Summary of Trends in Growth and Anemia Indicators by Race/Ethnicity, for the state obtain the race/ethnic specific prevalence of obesity for 1992 and list the prevalence defined as "B" in the table.

Sample: PedNSS Table 18C, Summary of Trends in Racial and Ethnic Distribution

Step 3

Multiply the proportion for each race/ethnic group for the state in 2001 (A) by the state obesity prevalence in 1992 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 in 1992. The table below shows this calculation for each race/ethnic group.

Summary

The adjusted prevalence for obesity for the state WIC program in 1992 is what the prevalence would have been based on the race/ethnic distribution of the state in 2001. The adjusted prevalence of obesity in 1992 is 9.8%, slightly higher than the original prevalence of 9.4% indicating that the change in the distribution of the population from 1992 to 2001 contributed marginally (0.4%) to the increase in overweight during this time period. Therefore, the change in the distribution of the population from 1992 to 2001 did not affect the prevalence of obesity .

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Page last reviewed: October 29, 2009
Page last updated: October 29, 2009
Content Source: Division of Nutrition, Physical Activity and Obesity, National Center for Chronic Disease Prevention and Health Promotion

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