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When are the health problems increasing or decreasing over time?

Surveillance data are analyzed by time and answer the question "when" to determine if health problems are increasing or decreasing over time.

  • The PedNSS and PNSS Summary of Trends in Health Indicators tables can be used to compare prevalence of health indicators for the most recent year of data to the previous year as well as examine the long term trend in prevalence of the health indicator.
  • Looking at only one year of data can be misleading because prevalence often varies from year to year. One population may have a higher prevalence one year, but a lower prevalence in either the previous or subsequent years. Trend data can provide a more accurate picture of whether or not the prevalence is changing over time.
  • Change in the prevalence of a health indicator over time may be the result of a change in the health indicator or simply a change in the distribution of a specific demographic characteristic (age, race/ethnicity, or education) of the population. The tables on Summary of Trends in Racial and Ethnic Distributions or Age can be used to determine if the population characteristics have remained stable or changed over time.
  • To determine if there is a true change in the prevalence of a specific health indicator and that the change is not due to a change in population characteristics such as age, race/ethnicity, or education you may want to conduct an additional standardization analysis to control for a specific person characteristic, thus, making the populations more comparable from one time period to another and making the prevalence of the health indicator of interest also comparable.

Learn how to calculate standardized rates using an example of standardization of race/ethnicity for a population over time.

Change in Prevalence of a Health Indicator Over Time

An increase or decrease in the prevalence of a health indicator or its trend over time can be portrayed either as absolute or relative change.

  • Absolute change is one prevalence value minus another.
    • Example: If the prevalence of obesity in 2001 was 12.9% and in 1992 it was 8.9%, the absolute change equals 12.9% minus 8.9% or an increase of 4.0%.
  • Relative change is the percentage change between the two years of interest and is calculated by first obtaining the absolute change or difference. This change or difference is divided by the prevalence of the first or earlier year to obtain the relative change which is the percentage of change over time.
    • Example: Divide the absolute change (4.0%) by the first year prevalence (8.9%) = .45 or 45% relative increase.

To determine if you should present the absolute or relative change depends on the point you want to make. A general rule of thumb is that when you are concerned about the magnitude of a public health problem, such as how many people are affected, it may be appropriate to give emphasis to absolute rather than relative changes.

On the other hand, relative differences are of interest when you want to look at the processes of causation, for example, to examine the effect of health care or of a supposed risk factor or protective factor on the occurrence of a health problem. It is not always easy to choose between the use of absolute and relative change and sometimes both are important. However, the most important question is whether the health problem is getting better or worse.

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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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