The proportion of people who become diseased during a specified period of time and calculated as:
CI = number of new cases of a disease during a given period of time / total population at risk
An even more precise estimate of incidence that utilizies all available information is the incidence rate (IR), force of mobility or mortality, or incidence density (ID). This is considered to be a measure of the instantaneous rate of development of disease in a population and is defined as:
ID = number of new cases of a disease during a given time period / total person-time of observation
See Epidemiology, Incidence, Odds ratio, Prevalence