incidence, in epidemiology, occurrence of new cases of disease, injury, or other medical conditions over a specified time period, typically calculated as a rate or proportion. Examples of incident cases or events include a person developing diabetes, becoming infected with HIV, starting to smoke, or being admitted to the hospital. In each of those situations, individuals transition from an occurrence-free state to an occurrence.

Incidence versus prevalence

Incidence contrasts with prevalence, which includes both new and existing cases. For example, a person who is newly diagnosed with diabetes is an incident case, whereas a person who has had diabetes for 10 years is a prevalent case. For chronic diseases, such as diabetes, a person can have an incident case just once in a lifetime. For diseases that can be resolved (e.g., the common cold), a person can have multiple incidences over his or her lifetime.

The study of incident cases provides information about the etiology (or cause) of a disease and its outcome. It also allows researchers to determine the risk factors for a disease or other medical condition. The study of prevalent cases, on the other hand, combines the study of new and surviving cases, making it unclear as to whether risk factors are the causes of new cases or causes of survival.

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Incidence proportion and incidence rate

Incidence can be measured as a proportion or as a rate. Measured as a proportion, it quantifies the risk of an occurrence in a given time period. Measured as a rate, it quantifies the number of new cases in a population over time. Thus, to calculate incidence, three elements must be defined: (1) the number of new cases, (2) the population at risk, and (3) the time period.

For incidence proportion, the numerator is the number of new cases of a disease or condition that occur during a given time period, while the denominator is the total population at risk during the defined study period. To accurately measure incidence proportion, all individuals at risk for the outcome under study must be followed during the entire study period (or until experiencing the outcome). Because complete follow-up is required to directly compute incidence proportion, it is usually only calculated for studies with a short follow-up period. As an example, on a seven-day cruise, 84 of 2,318 passengers report to the ship’s infirmary with gastrointestinal illness. The incidence of disease on the ship would equal 84 new cases of illness divided by 2,318 total passengers at risk, resulting in an incidence proportion of 4 percent over the seven-day period.

The incidence rate numerator is likewise the number of new cases. The denominator, however, is the total person-time, or the amount of time that all at-risk persons were observed. For example, the hypothetical incidence rate of breast cancer among women age 40 or older equals 32 women with breast cancer divided by 3,896 person-years (persons per year) of follow-up, which is equivalent to 821 per 100,000 at-risk persons per year.

Because incidence rate includes persons entering and leaving study populations, whereas incidence proportion assumes that those individuals were disease-free, it generally is more accurate than incidence proportion for long-term studies. Hence, an accurate measure of incidence, whether incidence proportion or incidence rate, requires a precise definition of the denominator. Because incidence is a measure of new cases during a given time period, it is important that those persons in the denominator be at risk. They should not have a history of the disease in question if it is a chronic disease, nor should they otherwise not be able to develop a new case of disease (e.g., women cannot get prostate cancer). An additional consideration for the use of incidence rate is that it assumes a constant probability of disease, which may not reflect actual probability, particularly for conditions whose risk increases with age.

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