Incidence rate or person-time rate is a measure of incidence that incorporates time directly into the denominator. Secondary attack rate = (17 ⁄ (86 − 18)) × 100% = (17 ⁄ 68) × 100% = 25.0% Incidence rate or person-time rate Definition of incidence rate If the 18 households included 86 persons, calculate the secondary attack rate.

One incubation period later, 17 persons in the same households as these “primary” cases developed shigellosis. If the population of the community was 1,000, then the overall attack rate was 18 ⁄ 1,000 × 100% = 1.8%. A variety of attack rates can be calculated.ĮXAMPLE: Calculating Secondary Attack RatesĬonsider an outbreak of shigellosis in which 18 persons in 18 different households all became ill. It is the risk of getting the disease during a specified period, such as the duration of an outbreak. In the outbreak setting, the term attack rate is often used as a synonym for risk.This is an example of field epidemiologists doing the best they can with the data they have. In fact, ideally the denominator should be restricted to women with ovaries, excluding women who have had their ovaries removed surgically (often done in conjunction with a hysterectomy), but this is not usually practical. This is easily accomplished because census data by sex are readily available. For example, if the numerator represents new cases of cancer of the ovaries, the denominator should be restricted to women, because men do not have ovaries. The denominator should be limited to the “population at risk” for developing disease, i.e., persons who have the potential to get the disease and be included in the numerator. The denominator of an incidence proportion is the number of persons at the start of the observation period. Because all of the persons with new cases of disease (numerator) are also represented in the denominator, a risk is also a proportion. The denominator is the number of persons in the population at the start of the observation period. As a measure of incidence, it includes only new cases of disease in the numerator. Incidence proportion is a measure of the risk of disease or the probability of developing the disease during the specified period.Risk = “Food-specific attack rate” = (30 ⁄ 99) × 100 = 0.303 × 100 = 30.3% Properties and uses of incidence proportions Numerator = 30 persons who ate potato salad and developed gastroenteritisĭenominator = 99 persons who ate potato salad Calculate the risk of illness among persons who ate potato salad. Numerator = 100 deaths among the diabetic menĮxample B: In an outbreak of gastroenteritis among attendees of a corporate picnic, 99 persons ate potato salad, 30 of whom developed gastroenteritis. Calculate the risk of death for these men. EXAMPLES: Calculating Incidence Proportion (Risk)Įxample A: In the study of diabetics, 100 of the 189 diabetic men died during the 13-year follow-up period.

Times table up to 100