Incidence refers to the number of new cases of a disease or condition that occur in a specified population during a defined period. It is a measure of the rate at which new events occur in a population. Incidence can be categorized into two types:
Incidence Rate: This is the number of new cases per unit of person-time at risk. It is calculated as:
Incidence Rate = (Number of new cases during a time period) / (Total person-time at risk during the time period)
Person-time at risk is the sum of the periods of time that all individuals in the population are at risk of developing the disease.
Cumulative Incidence: This is the proportion of a population that develops a condition over a specified period. It is calculated as:
Cumulative Incidence = (Number of new cases during a time period) / (Total population at risk at the beginning of the time period)
