An observed relative risk of 2.0 for smoking and lung cancer implies what?

• A. Smokers have half the risk of developing lung cancer compared with non-smokers.
• B. Smokers have twice the risk of developing lung cancer compared with non-smokers.
• C. There is no association between smoking and lung cancer.
• D. The result indicates a protective effect of smoking.

Answer: (B)

Understanding relative risk helps you see how much more (or less) likely an outcome is in the exposed group compared with the unexposed group. An observed relative risk of 2.0 means the incidence of lung cancer among smokers is twice that among non-smokers. In practical terms, if non-smokers have a certain baseline risk, smokers have about double that risk. For example, if non-smokers have 5 cases per 100,000 people per year, smokers would have about 10 cases per 100,000 per year.

This indicates a positive association between smoking and lung cancer in the data. Remember, a relative risk greater than 1 signals increased risk, equal to 1 signals no difference, and less than 1 signals a protective effect. The figure reflects a ratio, not an absolute probability, and statistical significance depends on the confidence interval not including 1.