These annotated slides will help you interpret an OR or RR in clinical terms. Please download these slides and view them in PowerPoint so you can view the annotations describing each slide.

1. What are the Results of
This Harm Study?
How strong is the association between
exposure and outcome? What’s the
risk?
Terry Shaneyfelt, MD, MPH
Associate Professor of Medicine
UAB School of Medicine
ebmteacher.com

2. Outcome measures of observational
studies
• Cohort Studies
– Relative risk or risk ratio (RR)
• Case-control Studies
– Odds ratio (OR)

3. Relative Risk (Risk Ratio)
• Expresses how many times more (or less)
likely an exposed person develops an
outcome relative to an unexposed person
• Interpretation:
– RR > 1 Increased risk of outcome
– RR = 1 No risk of outcome
– RR < 1 Reduced risk of outcome

4. What is the correct interpretation
of a RR of 1.36?
1. Risk of disease is increased by 136%
2. Risk of disease is reduced by 36%
3. Risk of exposure is reduced by 36%
4. Risk of exposure is increased by 36%
5. Risk of disease is increased by 36%
6. Risk of exposure is increased by 136%

5. 2 more
1. What is the interpretation of a RR of 0.80?
– Risk of the outcome in the exposed group was reduced
by 20% (or occurred 20% less) relative to the
unexposed group.
2. What is the interpretation of a RR of 3.30?
– Risk of the outcome in the exposed group was increased
by 230% relative to the unexposed group or the
outcome was 3.3 times more likely to occur in the
exposed group than in the unexposed group.

6. Don’t be fooled…It’s all relative
• RR = 3 (3 times more likely to develop the
outcome if you are exposed to X)
– Are you impressed?
– Yes if started at 30%  90%
– Not so much if start at 0.1%  0.3%
• Have to assess baseline risk of your patient
and then apply the RR

7. NEJM 1996;335:453

8. Odds Ratio
• How many times more likely the odds of finding
an exposure in someone with disease is compared
to finding the exposure in someone without the
disease
• Interpretation:
– OR > 1 Increased frequency of exposure among cases
– OR = 1 No change in frequency of exposure
– OR < 1 Decreased frequency of exposure

9. How big should the RR or OR be for
us to be impressed?
• In a RCT we would be satisfied with a small
increase (or decrease) in risk
• Cohort study: RR > 3 for a minor adverse event
• Case-control: OR > 4
From: EBM How to Practice and Teach It 4th ed. Pg 195

10. Prof Les Irwig’s Suggested rule of thumb
of when to trust the RR or OR
• Compare the unadjusted RR or OR to one in
which at least 1 known confounder was
adjusted out
• If adjustment produces a large decline in the OR
or RR be suspicious of a spurious association
• In contrast, if the adjustment increases the RR
or OR or if it remains stable then we can be
more confident in the validity of the association.
From: EBM How to Practice and Teach It 4th ed. Pg 195

11. NEJM 1996;335:453
Unadjusted RR = (8/27161) ÷ (431/304744) = 0.208