Consider a cohort of 2,000 people, of whom 800 are smokers and 1,200 are non-smokers. The entire cohort is observed for 20 years, and 100 participants develop lung cancer, of who 90 are smokers and 10 are not.

What is the incidence rate of lung cancer for non-smokers in this cohort per 1,000?

Question

Consider a cohort of 2,000 people, of whom 800 are smokers and 1,200 are non-smokers. The entire cohort is observed for 20 years, and 100 participants develop lung cancer, of who 90 are smokers and 10 are not.

What is the incidence rate of lung cancer for non-smokers in this cohort per 1,000?

BlackTom AI · Accepted Answer

First, identify the subgroup of interest: non-smokers. There are 1,200 non-smokers in the cohort.
Next, note the number of new cases of lung cancer among non-smokers over the 20-year follow-up: 10 cases.
To compute the incidence rate in terms of per 1,000 individuals over the 20-year period (cumulative incidence expressed per 1,000), use the formula: (number of new cases / number at risk) × 1,000.
Applying the numbers: (10 / 1,200) × 1,000 = 0.008333... × 1,000 = 8.333..., which rounds to 8.3 per 1,000.
Now evaluate the other options by checking what they would imply about the number of cases or the denominators:
- 42 per 1,000 would correspond to (42/1,000) × 1,200 = 50.4 ≈ 50 cases, which is not the observed 10 cases among non-smokers.
- 12.5 per 1,000 would correspond to (12.5/1,000) × 1,200 = 15 cases, not 10.
- 112.5 per 1,000 would correspond to (112.5/1,000) × 1,200 = 135 cases, far from 10.
- 8.3 per 1,000, as calculated above, matches the observed data (10 cases among 1,200 non-smokers) over the 20-year follow-up when expressed per 1,000 people at risk.
Therefore, the option that aligns with the data for non-smokers is the one that yields 8.3 per 1,000.

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