Are the results of this study important?
What is the magnitude and precision of the treatment effect?
The results of systematic reviews are commonly presented as odds ratios (OR) or relative risks (RR). The ratio of the odds of having the outcome of interest in the experimental group relative to the odds of that same event occurring in the control group is termed the odds ratio. The ratio of the risk of the outcome of interest in the experimental group relative to the risk of the same outcome in the control group is called the relative risk.
Like the relative risk reduction, neither the odds ratio nor the relative risks preserves the baseline risk of the outcome event, and, they are difficult to translate at the point of care. This can be overcome by converting them to a number needed to treat (NNT).
To calculate the NNT/NNH for any OR or RR we can use the following equations:
- For RR < 1:
- NNT = 1/(1-RR) x PEER
- For RR > 1:
- NNT = 1/(RR-1) x PEER
- For OR < 1:
- NNT = 1-[PEER x (1-OR)]/(1-PEER) x (PEER) x (1-OR)
- For OR > 1:
- NNT = 1 + [PEER x (OR-1)]/(1-PEER) x (PEER) x (OR -1)
Alternatively, (and more simply) we could use the tables below to obtain the NNT for specific OR and PEERs.
When OR < 1:
| For Odds Ratios LESS than 1 | ||||||||
|---|---|---|---|---|---|---|---|---|
| 0.9 | 0.8 | 0.7 | 0.6 | 0.5 | 0.4 | 0.3 | ||
| Patient Expected Event Rate (PEER) | 0.05 | 209 | 104 | 69 | 52 | 41 | 34 | 29 |
| 0.10 | 110 | 54 | 36 | 27 | 21 | 18 | 15 | |
| 0.20 | 61 | 30 | 20 | 14 | 11 | 10 | 8 | |
| 0.30 | 46 | 22 | 14 | 10 | 8 | 7 | 5 | |
| 0.40 | 40 | 19 | 12 | 9 | 7 | 6 | 4 | |
| 0.50 | 38 | 18 | 11 | 8 | 6 | 5 | 4 | |
| 0.70 | 44 | 20 | 13 | 9 | 6 | 5 | 4 | |
| 0.90 | 101 | 46 | 27 | 18 | 12 | 9 | 4 | |
When OR > 1:
| For Odds Ratios GREATER than 1 | ||||||||
|---|---|---|---|---|---|---|---|---|
| 1.1 | 1.25 | 1.5 | 1.75 | 2 | 2.25 | 2.5 | ||
| Patient Expected Event Rate (PEER) | 0.05 | 212 | 86 | 44 | 30 | 23 | 18 | 16 |
| 0.10 | 113 | 46 | 24 | 16 | 13 | 10 | 9 | |
| 0.20 | 64 | 27 | 14 | 10 | 8 | 7 | 6 | |
| 0.30 | 50 | 21 | 11 | 8 | 7 | 6 | 5 | |
| 0.40 | 44 | 19 | 10 | 8 | 6 | 5 | 5 | |
| 0.50 | 42 | 18 | 10 | 8 | 6 | 5 | 4 | |
| 0.70 | 51 | 23 | 13 | 10 | 9 | 8 | 7 | |
| 0.90 | 121 | 55 | 33 | 25 | 22 | 19 | 18 | |
Returning to our clinical scenario, we are interested in the outcome death and dependency. The OR for these events is 0.75 (95 % CI 0.65 to 0.87). From the review, we've obtained a PEER for death and dependency of 68% and using the table above, we can estimate that the NNT is approximately 20.