Prevalence

In epidemiology, the term prevalence quantifies the proportion of a population with disease or a particular condition at a specific point in time (sometimes called point prevalence). Prevalence is used widely in the media and by government agencies, insurance companies, epidemiologists, and health care providers. Often confused with prevalence, incidence (described in [Page 838]detail elsewhere) quantifies new cases while prevalence describes existing cases.

While the term prevalence rate is often used synonymously with prevalence, the strict definition restricts prevalence to a proportion, not a rate. The difference is in the denominator: Rates describe risk of disease during a given time interval among a population at risk, while proportions describe the likelihood of disease at a specific point in time among the population. The point in time may be a specific calendar date or a time that varies from person to person, such as the onset of menopause or puberty, or discharge from the hospital.

For prevalence, the numerator is the number of existing cases or conditions, and the denominator is the total population or group. For example, the prevalence of type 2 diabetes among children aged 2 to 12 years equals the number of children aged 2 to 12 years with type 2 diabetes divided by the total number ofchildrenaged2to12years.

While incidence helps investigators understand the etiology (or cause) of disease, prevalence is especially useful to health system planners and public health professionals. Knowledge of the disease burden in a population, whether global or local, is essential to securing the resources required to fund special services or health promotion programs. For instance, the director of a nursing home must be able to measure the proportion of seniors with Alzheimer's to plan the appropriate level of services for the residents. Legislators and public health professionals need good population statistics to prioritize funding for health promotion programs, such as obesity and smoking cessation. On a community level, understanding the prevalence of English as a second language would be helpful to school administrators. Nationaland statelevel prevalence of behaviors and diseases are usually calculated using data collected systematically from the population through major health surveys, such as the CDC's Behavioral Risk Factor Surveillance Survey (BRFSS), the National Health Interview Survey (NHIS), and National Health and Nutrition Examination Survey (NHANES).

Understanding the difference between prevalence and incidence allows the epidemiologist to apply the terms correctly, define denominators for measures, and conceptualize the study design best suited to a specific research question. In addition, these terms are related mathematically, a property that can prove useful when moving from a measure of incidence to prevalence or vice versa. When the incidence of disease is stable over time, such as in the absence of epidemics or changes in treatment effectiveness, prevalence is the product of the incidence and the average duration of disease or condition (P = I × D). More complex mathematical relationships exist between incidence and prevalence when these assumptions cannot be met.