Posted: September 11th, 2016

The risk of developing stroke in relation to hypertension, expressed by the risk ratio and the odds ratio. Compare the 2 results, and decide which one is more appropriate in this study.

A prospective cohort study is run to estimate the incidence of stroke in persons 55 years of age and older. The investigators recruited 1000 participants, 300 with hypertension and 700 without hypertension. All participants are free of stroke at study start. Each participant is followed for a maximum of 5 years. Assume, that there were no drop-outs or deaths among all participants.
developed stroke during the 5 years follow-up period didn’t develop stroke during the 5 years follow-up period sum of stroke-free person years for the participants who developed stroke
hypertension 27 273 3×1 + 5×1.5 + 4×2 + 8×3 + 4×3.5 + 3×4.5
= 70 person years
no hypertension 32 668 2×1 + 3×1.5 + 3×2 + 2×2.5 + 4×3 + 5×3.5 + 8×4 + 5×4.5
= 101.5 person years
N. B. Stroke-free person years are the number of years that a person stays healthy without stroke. For those who developed stroke, this number is calculated by multiplying the number of persons by the years they have stayed healthy before having the stroke. The sum for those stroke-free person years is presented in the table. For the rest of the participants who never developed stroke during the 5 years, the stroke-free person years are the products of their number by the 5 years of follow up.
Please calculate the following:
a) The cumulative incidence of stroke at 5 years
b) The incidence rate of stroke at 5 years
c) The incidence rate of stroke among people with hypertension at 3 years
d) The risk of developing stroke in relation to hypertension, expressed by the risk ratio and the odds ratio. Compare the 2 results, and decide which one is more appropriate in this study.