Incidence Rate Question

Question

1. What is the cumulative incidence and incidence rate of cancer in decimal form?

Participant A:

• Age: 7

• Outcome: Cancer

Participant B

• Age: 2

• Outcome: No Cancer

Participant C

• Age: 2

• Outcome: Cancer

Participant D

• Age: 5

• Outcome: No Cancer

Participant E:

• Age: 9

• Outcome: Cancer

Participant F:

• Age: 11

• Outcome: Cancer

Participant G:

• Age 9

• Outcome: No Cancer

Participant H:

• Age: 5

• Outcome: No Cancer

For the incidence rate I calculated this as:

4/8= 0.5

Cumulative Incidence Rate:

0.5 • 100,000 = 50,000

I think I did this wrong but any help would be appreciated.

Answer 1

Cumulative incidence is calculated as: (# of new cases during observation period) / (# of cases at-risk during observation period).

To determine the numerator, count the number of participants in your sample who developed cancer. In this case, it is 4. To determine the denominator, count all participants who were at-risk of developing cancer at the beginning of the study. For this question, we can assume all 8 participants were at-risk at the beginning of the study, so the denominator is 8. Therefore, cumulative incidence is 4/8, or 0.5. Your calculation is correct.

Incidence rate is calculated as: (# of new cases during observation period) / (amount of person-time of follow-up).

The numerator is the same as the numerator for cumulative incidence (i.e., 4). To determine the denominator, add up the years of observation for ALL participants (i.e., for those who developed cancer and those who did not develop cancer). Based on the information provided, it looks like we can assume each participant was followed from birth until the age provided, so the years of observation for each participant is equal to their age in years. So Participant A was contributed 7 years of observation, Participant B contributed 2 years of observation, etc. Adding this for all participants gives: 7 + 2 + 2 + 5 + 9 + 11 + 9 + 5 = 50 person-years of observation. Therefore, incidence rate = 4 cases / 50 person-years = 0.08 cases per person-year.