Calculating Relative Risk in a Cohort Study
The relative risk (RR) is 6.0, calculated by dividing the incidence rate in smokers (600/5000 = 12%) by the incidence rate in non-smokers (100/5000 = 2%).
Step-by-Step Calculation
Incidence Rates in Each Group
- Smokers: 600 cases out of 5000 exposed individuals = 0.12 or 12% incidence
- Non-smokers: 100 cases out of 5000 unexposed individuals = 0.02 or 2% incidence
Relative Risk Formula
RR = (Incidence in exposed group) / (Incidence in unexposed group)
RR = (600/5000) / (100/5000) = 0.12 / 0.02 = 6.0
Clinical Interpretation
This RR of 6.0 indicates that smokers in this cohort had 6 times the risk of developing breast cancer compared to non-smokers.
Context from Medical Literature
- This calculated RR of 6.0 substantially exceeds the typical associations reported in the literature for smoking and breast cancer risk 1, 2, 3
- Published studies generally report more modest associations, with relative risks ranging from 1.25 to 1.48 for active smokers compared to never smokers 1, 3, 4
- The National Comprehensive Cancer Network guidelines note that smoking is a weaker risk factor for breast cancer compared to other established risk factors 5
Important Caveats
- This hypothetical RR of 6.0 is unusually high compared to real-world epidemiological data on smoking and breast cancer 1, 2, 3
- The actual relationship between smoking and breast cancer shows increased risk particularly among women who started smoking before first pregnancy, before menarche, or as teenagers 3
- Studies with more complete exposure assessment (including passive smoking) report pooled relative risks around 1.90 for exposed non-smokers and 2.08 for active smokers 2
- One large prospective study found no material association between smoking and breast cancer risk (RR 1.02 for heavy smokers) 6