Period prevalence

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In epidemiology, Period prevalence is the proportion of the population with a given disease or condition over a specific period of time. It could describe how many people in a population had a cold over the cold season in 2006, for example.

It is expressed as a percentage of the population and can be described by the following formula:

Period prevalence (ratio) = Number of cases that occurred in a given period ÷ Number of people in the population during this period

The relationship between incidence (rate), point prevalence (ratio) and period prevalence (ratio) is easily explicated via an analogy with photography. Point prevalence is akin to a flashlit photograph: what is happening at this instant frozen in time. Period prevalence is analogous to a long exposure (seconds, rather than an instant) photograph: the number of events recorded in the photo whilst the camera shutter was open. In a movie each frame records an instant (point prevalence); by looking from frame to frame one notices new events (incident events) and can relate the number of such events to a time period (number of frames) -> incidence rate.

PEDANTIC NOTE

Contrary to the opinion of those not blessed with an English education it is important to distinguish between the notions of ratio and rate. A ratio is one number divided by another. A rate entails reference to the passage of time: miles per hour, new cases per year (incidence), etc. A rate is necessarily a ratio but not vice versa.

See also

• Point prevalence

References

Gerstman, B.B. (2003). Epidemiology Kept Simple (2nd ed.). Hoboken NJ: Wiley-Liss.