Question

1. Consider the data in the following table, obtained from a cohort study conducted by Iso and colleagues [Isa H, Date C, Yamamoto A, et al. Smoking cessation and mortality from cardiovascular disease among Japanese men and women. The JACC study. Am J Epidmeiol. 2005;161(2):170-179.].

Total Cardiovascular Disease According to Smoking Status

	Disease
Current Smoker	Cases		Person-years
Yes	882	—	220,965
No	673	—	189,254

Calculate the rate ratio and corresponding 95% confidence interval for these data. Express your answer to the nearest hundredth. Interpret your results.

Answer 1

Rate ratio is incidence rate in a exposed group divided by the incidence rate in an unexposed group

Here exposed group is a current smoker , the incidence rate is =882/220965=0.003992

incidence rate for an unexposed group is 673/189254=0.003556

Hence RR= 1.1225

To calculate Confidence interval we will first find out natural logarithm of RR ie LRR=ln RR=0.1156

Next step is to calculate Standard error of LRR =S.E(LRR)=

Next is to calculate [LRR-1.96*S.E(LRR),LRR-1+96*S.E(LRR)]=(0.1156-1.96*0.051,0.1156+1.96*0.051)=(0.01564,0.21556)

Then calculate the antilogarithm of this interval ie which is the 95% confidence interval of Incidence rate ratio

In this study, current smoker had 1.1225 times the rate of developing cardiovascular disease compared to a non smoker. Based on this sample, we are 95% confident that the true rate ratio lies between 1.02 and 1.24. (The answers were rounded to two decimals)