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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