Minimum sample sizes required for improvement projects based on observed and desired system performance
| Observed system performance (%) | Desired system performance | |
|---|---|---|
| 80% | 90% | |
| 95 | 26 | 140 |
| 90 | 70 | Not applicable |
| 85 | 260 | 180 |
| 80 | Not applicable | 50 |
| 75 | 280 | 28 |
| 70 | 80 | 20 |
| 66 | 45 | 15 |
| 60 | 25 | 10 |
| 50 | 12 | 6 |
| 40 | 10 | 5 |
| 20 | 5 | 5 |
The table shows the approximate sample size required to reject the null hypothesis that observed performance (from an audited sample) is consistent with the desired system performance, shown here as being either 80% or 90%. If you wish to calculate an exact p value for your audit or Plan–Do–Study–Act (PDSA) result, follow the steps in online supplementary appendix 1. If you wish to calculate the exact 95% CI for your audit or PDSA result, follow the steps in online supplementary appendix 2. The results shown here all use the conventional two-tailed p value of 0.05.