Disease Incidence Rate calculation

Question

A group of five men were studied over a period of 5 years. The following information shows the results of the study:

Subject A: lost to follow up after 2 years

Subject B: Developed disease at 3 years of follow-up

Subject C: followed for 5 years, did not develop disease

Subject D: followed for 4 years, did not develop disease

Subject E: followed for 2.5 years, developed disease at 2.5 years.

What is the incidence rate of this disease?

Answer 1

To calculate an incidence rate ratio (IRR), we need to divide the number of people who developed the disease (numerator) by the total number of person-years of follow-up (denominator). We can determine each of these as follows:

Numerator: We see from the description that 2 people developed the disease.

Denominator: We calculate that there were 16.5 person-years of follow-up as shown below.

Subject A: 2 years

Subject B: 3 years

Subject C: 5 years

Subject D: 4 years

Subject E: 2.5 years

Total person-years: 2 + 3 + 5 + 4 + 2.5 = 16.5

Putting this together:

IRR = 2 cases / 16.5 person-years = 0.121 cases per person-year

We can also report this as 12.1 cases per 100 person-years or 121 cases per 1000 person years.

Note this is the IRR among these 5 men, over a period of 5 years.