Incidence quantifies the number of new events or cases of disease that develop in a population of individuals at risk during a specified time interval.
This is an estimate of the probability that a patient in a given population at risk will develop the condition during a specified period of time. For example, the incidence of bowel obstruction in a population of patients with colorectal cancer followed for 30 months from the date of diagnosis.
Incidence can be summarized as:
Number of new cases / total population at risk
Sand MB, Piza M, Ingham JM. The epidemiology of end-of-life experience. Hanks G, Cherny NI, Christakis NA, Fallon M, Kaasa S, Portenoy RK (Ed).Oxford Textbook of Palliative Medicine. 4th ed. Oxford University Press, 2010. pp. 58-80.