Example
Say that 14 events are observed in 200 people studied for 1 year and 100 people studies for 2 years.
The person time at risk is 200 + 100 x 2 = 400 person years
For this example:
Events observed = 14
Time at risk of event = 400
Poisson (e.g. incidence) rate estimate = 0.035
Exact 95% confidence interval = 0.019135 to 0.058724
Here we can say with 95% confidence that the true population incidence rate for this event lies between 0.02 and 0.06 events per person year.