Question

Dr. Wohl, I read many reports that use statistical numbers I'm not familiar with. For example, 1.1 sero-conversions per 100 person years. What exactly does this mean? Is it 100 persons or something else?

Answer

What is being reported here is an incidence rate. This is used to look at new cases of whatever you are interested in when there are varying periods of observation in the group being studied. For instance some people in the group are followed for longer period and some shorter periods. The number of new cases is described as being per the total person time at risk.

So in the example you gave, there were 1.1 new HIV cases in per 100 years that the group members added together were at risk.

Another way to think about this would be if you looked at a group of motorists to see how many accidents occurred during study. You can count the accidents and express this per motorist but that ignores the fact that different drivers drove far and some very little. Better would be to see how many accidents occurred per mile driven. This could be 1.1 accidents for every 100 miles a person drove.

If there are any epidemiologists out there, please feel free to elaborate and/or correct. DW