 # Measures of Morbidity: Incidence Rate and Attack Rate

When all at risk people are not observed for the entire time period

This situation results when individuals migrate, die (due to other causes), are lost to follow-up, etc. Since the time under observation is not uniform, we calculate how much time each person was at risk and observed. This is called person-time, and may be expressed as person-months or person-years (py) of observation. The incidence rate thus calculated is also called incidence density, and the denominator is the sum of all person-times.

Let us consider person-years (py) in some detail:

If one person at risk is observed for one year, the person-time is equal to 1 person-year.

By extension, two persons at risk observed for one year yield 2 person-years of observation. One person observed for two years is also = 2 person-years. Four persons observed for 6 months each is also = 2 person-years.

Illustration:

Four persons were observed for four years, and the duration of observation for each is shown below:

Person-years are calculated for each year:

Year 1: All four persons were under observation. Therefore, we have 4 person-years of observation.

Year 2: Person 2 and 3 only were under observation. Therefore, we have 2 person-years of observation.

Year 3: Person 2 and 3 alone were under observation. Therefore, we have 2 person-years of observation.

Year 4: Only person 3 was under observation. Therefore, we have 1 person-year of observation.

The total person-time is the sum of person-years for each year: 4+2+2+1= 9 person-years.

If person 1 and 4 became cases after 1 year, then the incidence rate per 1,000 is calculated as:

(No. of new cases during specified time period/ Total person-time) *1,000 =2/9 *1,000 person-years

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