Example: Understanding the incidence and prevalence rate of public health concerns
Two important types of data you might want to collect include information on the incidence and prevalence on the problem. Example One explains these terms, then shows how you might graphically illustrate this data in tables.
The rate at which something occurs is simply a quantity measured in terms of another measured quantity. For example, the rate of rape in the United States in 1992 was 42.8 cases (first quantity) per 100,000 women (second quantity) (U.S. Department of Justice, 1993). The terms proportion and ratio are often used interchangeably for rate, and the information may be written as a percentage.
The incidence rate tells how many new cases of a problem arise in a specific period of time. And because you often can't talk about how many new cases of a problem there are today (you wouldn't usually say, for example, that 27 members of the community became alcoholics last Thursday), incidence rate is generally measured over a longer period of time, with one year being the average. For example, the Public Health Service has documented that each year (time period), 20,000 people (new cases) die from homicide. This is the incidence rate for homicide.
The prevalence rate, on the other hand, determines the likelihood that an individual will have a given problem at any particular moment in time. This helps determine the overall magnitude of the problem. An example of prevalence is: 26% of Americans have high blood pressure (U.S. Department of Health and Human Services, 1994). These may or may not be new cases. Some of these people probably had high blood pressure last year, some probably didn't. But when you are talking about cardiovascular disease, you can safely say that approximately a quarter of Americans currently have high blood pressure. This is the prevalence rate for high blood pressure.
As you can see, incidence and prevalence rates measure different things. Often, your organization will want to determine both in your research. You might find the prevalence rate valuable when you are talking about how widespread the problem you are dealing with is, and use the incidence rate to show your coalition has reduced the number of new cases, or the age of onset of the problem.
Another example of where you might want to measure both the incidence and prevalence rate of a problem is alcoholism. You can talk about the prevalence of alcoholism in your community, as well as the incidence of alcohol related deaths in the last year.
The following two tables are examples of the incidence of death (by particular causes) and the prevalence rate of selected risk factors for heart disease in one Kansas county. Note that in both cases, the information is put in a larger context. The incidence rate is compared to both state and national levels, and is clearly marked, out of every 100,000 people; the prevalence rate is given as a percentage, and is compared to that of the nation as a whole.
In the first table, the rank is given to show which was the leading cause of death in Wilson County, in Kansas, and in the United States. The viewer can then compare easily what causes a disproportionate number of deaths in their county. For example, the third leading cause of death in the county is unintentional injury. That's higher than in the rest of the state or in the country as a whole. (In both categories, it was the fifth leading cause of death). Community members might rightly decide, then, to try to reduce the number of unintentional injuries by starting a Safety First campaign.
Table 1: Incidence of death ranked by rate per 100,000
|Cause of death||Wilson County||Kansas||United States|
|Chronic Obstructive Pulmonary Disease||4||33||4||45||4||39|
|Nephritis, Nephrotic Syndrome, and Nephrosis||9||7||9||13||9||10|
|Ulcer of the stomach and duodendum||10||2||--||--||--||--|
Sources of information:
- Death rate for county based on age-adjusted annual average for 1987-1991. (Kansas County Health Profile 1994: Wilson County, KDHE).
- Death rate for state based on mortality data and population estimate. (Annual Summary of Vital Statistics, KDHE, 1993.)
- Death rate for nation from estimates from National Center for Health Statistics and based on 10 percent sample. (Annual Summary of Vital Statistics, KDHE, 1993.)
In table 2, the percentage of people who have risk factors for heart disease have been ranked for the State of Kansas and the nation as a whole. For example, 26% of Kansans are obese, compared to 24% of people in the nation as a whole.
Table 2: Prevalence of selected risk factors for heart disease, 1992
1992 Behavioral Risk Factor Surveillance System, KDHE