Question

Explain the following measures of incidence and compare their applications: incidence rate, cumulative incidence rate, incidence density, and attack rate.

Answer 1

• Incidence rate is defined as the number of new cases occurring in a defined population during a specified time period.
• No of new cases of specific disease during a given time period /population at risk during that period. It is often used to measure the occurance of acute or short term disease with exactly defined onset such as acute intestinal disease, poisoning, car accident, stroke etc.

• Cumulative incidence rate is number of new cases in a specific time period divided by number of population that is at risk of the disease.
• CI= No. of new cases during a specified period of time /no. of total population at risk
• Eg: 80 new cases of cervical cancer in 5 year period /4000women at risk =0.02=20 cases for 1000population
• Incidence density rate is the speed at which a defined at risk group or population develops a new clinical condition or outcome over a given time period.
• IDR = no.of newly diseased individual or cases /sum of time periods for all disease -free individual at risk.
• Attack rate is incidence rate which are calculated narrowly defined population during limited period of time such as an epidemic.