Question

Suppose we observe 20 cases of prostate cancer among 5,000 men ages 60-79 over a 10 year period. Of the men followed over time, 20 were followed for 5 years, 15 were followed for 8 years, 15 were followed for 3 years, and the rest were followed for 10 years. Calculate the incidence rate (per 1,000). (5 pts)

Answer 1

Note that incidence rate measures frequency of disease occurance in a population at risk over a given time period.

The formula for calculating the incidence rate is

Now in our example summed ‘person-years’ are

                = 20*5 + 15*8 + 15*3 + (5000-(20+15+15))*10  

(This is because, Of the men followed over time, 20 were followed for 5 years, 15 were followed for 8 years, 15 were followed for 3 years, and the rest were followed for 10 years.)

= 20*5 + 15*8 + 15*3 + (5000-50)*10

=    49765 person-years.

Number of new cases of disease (prostate cancer) in specified time interval = 20   (given)

Therefore,

Hence incidence rate of prostate cancer per 1000 is 0.04919