Question

Q3: A study followed a population of 2000 women aged over 65 years for 10 years and measured the number of cases of osteoporosis diagnosed during that time period. The investigators were interested in the effect of regular exercise on the development of osteoporosis and divided he women into two groups: 1000 women who took regular exercise and 1000 women who did not take regular exercise. The investigators recorded 800 new cases of osteoporosis over the 10 years of the study, 300 in those who took regular exercise and 500 in those who did not. The number of person years at risk was 8350 in those who exercised regularly and 6950 in those who did not. Calculate (i) the risk ratio and (ii) the rate ratio for the effect of regular exercise on osteoporosis in these women. Interpret your answer. (4pts)

Answer 1

1). The relative risk or risk ratio is the way to compare the risks for the two groups. To obtain the risk ratio, simply divide the cumulative incidence in the exposed group by the cumulative incidence in the unexposed group.

The incidence risk of   osteoporosis in exposed group/ those who exercise regularly = 300/1000= 0.3 per 10 years

The incidence risk of osteoporosis in non-exposed group/ those who do not exercise regularly= 500/1000=0.5 per 10 years

Risk ratio= cumulative incidence in exposed group / cumulative incidence in the unexposed group

Risk ratio= 0.3/0.5 = 0.6

The risk ratio of 0.60 indicates that the women over 65 years and exercise regularly have a lower level, 40% lower risk of developing osteoporosis than the women over 65 and do not exercise regularly in the cohort study population.

2) Rate ratio / Incidence ratio

The incidence rate of osteoporosis in women who exercised regularly (exposed) = 300/8350 person-years

= 0.03593 per person-year

= 35.93 per 1000 person-year

The incidence rate of osteoporosis in women who do not exercise regularly (unexposed)

= 500/6950 person years

=0.07194 per person-year

= 71.94 per 1000 person-years

Rate ratio= incidence rate in exposed group/ incidence rate in the unexposed group

=35.93/71.94= 0.50

The rate ratio of 0.50 represents that women over 65 years and who exercise regularly have 50% of the lower rate of osteoporosis than the women who do not exercise regularly.