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In medical papers, it is very common to calculate incidence rate and mention it's unit as per 100 person years. What does this means?

I know that incidence rate is the amount of people got the event per time but what does this person years interpreted?

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Let's assume you followed 100 people. 50 of them you followed for 2 years and nothing happened. 25 newly developed the disease of interest after being observed for on average 1 year (not counting time after developing the diagnosis). 25 could not be observed for the full two years (on average you observed them for 0.5 years).

In that case you observed 25 cases in 137.5 (=2×50+1×25+0.5×25) patient years of follow-up or 18.18 (=25/137.5×100) per 100 patient years.

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