The concept of incidence refers to the frequency of a new event in a population, such as onset of disease, occurrence of death, or initiation of a given medication.1 An incidence proportion is also known as risk, and it may be calculated as: Risk=A/N, where A is the number of subjects developing a disease (or experiencing an event) and N is the number of subjects followed for a period of time.4 Risk represents the probability of a new event occurring. To consider a hypothetical example, if 300 of 30,000 persons 65 and older experienced a heart attack during a one-year period, the risk of heart attack may be calculated as 300/30,000 or 1% for this population over a period of one year.
The incidence proportion or risk can only be meaningfully interpreted in reference to the amount of time subjects were followed (such as one year in the example above), although the amount of time subjects are followed is not part of the calculation of risk.4
A practical difficulty to calculating risk of an outcome such as heart attack in a group of subjects over time is that patients may die of unrelated causes such a motor vehicle accidents during follow-up. “Competing risks” is the term used to describe this phenomenon.2,4
A similar challenge is referred to as “loss to follow-up.”4 This occurs when the researcher can no longer track the patient as part of the study, which may arise due to the subject moving to a different jurisdiction or for other reasons.