Relative risk and odds ratio study guide

Relative Risk (RR)

The ratio of the probability of an event occurring in the exposed group versus the unexposed group. Formula: $\displaystyle RR = \\frac{P(E)}{P(U)}$.

Odds Ratio (OR)

The ratio of the odds of an event occurring in the exposed group to the odds in the unexposed group. Formula: $\displaystyle OR = \\frac{Odds(E)}{Odds(U)}$.

True or False: RR can exceed 1.

True. RR can exceed 1, indicating a higher risk in the exposed group.

True or False: OR cannot be less than 1.

False. OR can be less than 1, indicating lower odds in the exposed group.

When to use Relative Risk?

Use RR when you have cohort studies or clinical trials where incidence rates can be calculated.

When to use Odds Ratio?

Use OR in case-control studies where outcomes are known but not the incidence rates.

Relative Risk Interpretation

RR = 1: no difference; RR > 1: increased risk; RR < 1: decreased risk.

Odds Ratio Interpretation

OR = 1: no difference; OR > 1: increased odds; OR < 1: decreased odds.

Fill in the blank: RR compares _____ versus _____.

RR compares probability of event in exposed versus unexposed.

Example of RR calculation

If 30 out of 100 smokers develop lung cancer (30%) and 10 out of 100 non-smokers do (10%), RR = \\frac{30/100}{10/100} = 3.

Example of OR calculation

If 30 smokers and 10 non-smokers develop lung cancer, calculate odds: Smokers: 30/70, Non-smokers: 10/90. OR = \\frac{(30/70)}{(10/90)}.

Key difference between RR and OR?

RR measures risk directly; OR measures odds, which can be misleading for common events.

True or False: OR can approximate RR for rare events.

True. For rare events, OR approaches RR making them similar in interpretation.

How is RR calculated?

RR = \\frac{Incidence in exposed}{Incidence in unexposed}.

How is OR calculated?

OR = \\frac{(a/b)}{(c/d)} where a, b, c, d are counts in 2x2 table.

Relative Risk vs. Odds Ratio: Key Differences

Relative Risk (RR) measures the probability of an event occurring in exposed vs. non-exposed groups. Odds Ratio (OR) compares the odds of an event occurring in two groups. - RR is a probability; OR is a ratio of odds. - RR is often more intuitive for clinicians.

Relative risk interpretation

Relative risk (RR) is the ratio of the probability of an event occurring in the exposed group versus the non-exposed group. RR > 1 indicates increased risk; RR < 1 indicates decreased risk.

Odds ratio definition

The odds ratio (OR) compares the odds of an event occurring in the exposed group to the odds in the non-exposed group. OR > 1 indicates greater odds of the event in the exposed group.

True or False: RR and OR are the same.

False. RR compares probabilities, while OR compares odds. They may yield similar results, but they are not interchangeable.

Calculate RR: 40 smokers vs. 10 non-smokers develop cancer.

RR = (40/(40+60)) / (10/(10+90)) = 0.4 / 0.1 = 4.0. Smokers have 4 times the risk.

When to use odds ratio

Use OR when: - Study design is case-control. - You need to estimate the strength of association. - Data are expressed as odds rather than probabilities.

Relative Risk formula

RR = \\frac{P(E | G)}{P(E | NG)} where P(E | G) = probability of event in group, P(E | NG) = probability in non-group.

Fill in the blank: If RR = 2, the risk is ___ in the exposed group.

twice as high

How to interpret RR > 1?

Indicates increased risk in the exposed group compared to unexposed. - Example: RR = 2 means double the risk.

True or False: OR can estimate RR accurately in rare diseases.

True. Odds ratio approximates relative risk when the outcome is rare, providing similar interpretation.

Calculate OR: 30 smokers and 10 non-smokers develop lung disease. What’s OR?

OR = (30/70) / (10/90) = 3.86. Smokers are about 3.86 times more likely to develop lung disease.

Fill in the blank: A high RR suggests a strong ___ relationship between exposure and outcome.

causal - A strong association indicates potential causation.

Comparison: Use of RR vs OR in cohort studies.

RR is used for cohort studies; OR is often used for case-control studies. - RR gives direct risk, while OR evaluates odds of exposure.