Question

A five-year prospective cohort study has just been completed. The study was designed to assess the association between supplemental vitamin A exposure and mortality and morbidity for measles. The RR for measles mortality was 0.5. what is the mosrt appropriate conclusion about the association between the exposure and the disease of intrest:

a) the exposure is a risk factor for the disease

b) there is no statically significat association between the exposure and the disease

c) the exposure is protective aganist the disease

Answer 1

Answer: c) the exposure is protective against the disease

The risk ratio (RR) or relative risk in epidemiology, is the ratio of the probability of an outcome in an exposed group to the probability of an outcome in an unexposed group. It is the way to compare the risks for the two groups.

It is computed as the comparison between incidence in the exposed group and the incidence in the unexposed group.

Interpretation of RR

·         RR = 1, the exposure does not affect the outcome

·         RR <1, the risk of the outcome is decreased by the exposure

·         RR > 1, the risk of the outcome is increased by the exposure

In the case, the RR for measles mortality was 0.5, which is <1, so it is evident that the risk of the outcome is decreased by the exposure. Exposure to vitamin A appears to protect against mortality for measles.

The RR is obtained by simply dividing the cumulative incidence in exposed group by the cumulative incidence in the unexposed group.

RR = cumulative incidence in exposed group / cumulative incidence in the unexposed group.