Incidence

Incidence quantifies the number of new cases of a disease or events occurring within a specified time period (often one year) within a defined population who are at risk of that disease or event. It is usually expressed as a rate. Related concepts include:

• Cumulative incidence: This describes the proportion of individuals within a population at risk who develop the disease or experience the event over a specified period, often aggregated over a number of years.
• Incidence density (sometimes called force of morbidity or mortality): This is a more precise concept where individuals who develop the disease are removed from the eligible population as they are no longer at risk. The denominator for incidence density, therefore, becomes the aggregated person-time of eligibility (‘person-years of exposure’), rather than the number of individuals eligible at the start.

Specific types of incidence rates include:

• Mortality rate: The number of deaths in a population.
• Morbidity rate: The number of non-fatal disease cases in a population.
• Case fatality rate: The proportion of deaths within a population already suffering from a specific disease.
• Attack rate: The proportion of cases of a disease within a population already suffering from a specific disease.

It is important to note that direct comparisons of incidence rates can be misleading if there are differences in risk factors, such as age distribution, between the populations being compared. In such instances, statistical techniques such as standardisation (often used for mortality comparisons) or regression analysis may be used to adjust for these known differences.