Review Question - QID 217482

Relative Risk = (Probability of event in one group) / (Probability of event in other group). Therefore, in the example above, the relative risk would be calculated by W/(W + X) / Y/(Y+Z).

Risk is the probability of occurrence of an event or an outcome. Statistically, this is the chance of the outcome divided by all possible outcomes. The relative risk is the ratio of the risk of an event in one group compared to the risk of the event in the other group. Thus, relative risk provides an increase or decrease in the likelihood of an event based on some exposure/outcome. The relative risk does not provide any information about the absolute risk of the event occurring, but rather the higher or lower likelihood of the event in the exposure (in this case the anterior approach group) versus the non-exposure group (i.e. the posterior approach group). In the case above, the relative risk of wound dehiscence in the anterior versus the posterior approach group is calculated by the risk of wound dehiscence in the anterior approach group divided by the risk of wound dehiscence in the posterior approach group, which is W/(W + X) divided by Y(Y+Z). A relative risk <1 indicates a decreased risk in that group and >1 indicates a higher risk.

Ranganathan et al. provide a statistical review with a focus on the meaning of risk and odds and the difference between the two. They report "Odds" and "Risk" are the most common terms which are used as measures of association between variables.

Tenny et al. provide a concise review on relative risk. They note that relative risk is a ratio of the probability of an event occurring in the exposed group versus the probability of the event occurring in the non-exposed group. For example, the relative risk of developing lung cancer (event) in smokers (exposed group) versus non-smokers (non-exposed group) would be the probability of developing lung cancer for smokers divided by the probability of developing lung cancer for nonsmokers.

Figure A is a classic two-by-two table used for calculating different statistical values.

Incorrect Answers:

Answer 1: This would calculate the odds ratio. An odds ratio (OR) is a measure of association between an exposure and an outcome. The OR represents the odds that an outcome (i.e. wound dehiscence) will occur given a particular exposure (i.e. anterior approach), compared to the odds of the outcome occurring in the absence of that exposure. Using the above example, the odds ratio is calculated by the odds of the event in the exposure group (i.e. anterior approach) (W/Y) divided by the odds of the event in the other group (X/Z).
Answer 3: This is the calculation for relative risk reduction, which is the proportion of the risk reduction attributable to one intervention as compared to another.
Answer 4: This is the calculation for absolute risk reduction, which is the difference in risk (not the proportion) attributable to the intervention as compared to another group.
Answer 5: This is the calculation for attributable risk, which is the difference in risk between one group and another, or the proportion of an outcome occurrence that is attributable to the intervention.