# what is x per 100 person years mean?

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?

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.

• How observed 0.5 years? 25 could not be observed for the full two years (on average you observed them for 0.5 years) Commented Apr 3 at 12:18
• That's in the sense of "I know they each did not have an event after 0.5 years, but I don't know what happened to them afterwards." Or did you mean to ask how one defines "knowing"? That's easiest, if you constantly observe something (e.g. machine constantly filmed by camera so we know precisely the time it breaks down/how long we watched without it happening). It's harder if you only find out that patients were hospitalized since your conversation (might only take place every 6 months), but there's some possibility that you might find out between contacts (without this last bit, still easy). Commented Apr 4 at 12:37