Calculation of the number needed to screen for a hypothetical highly penetrant gene among subjects exposed to arsenic
| Gene | ||||
|---|---|---|---|---|
| Wild-type | Variant | |||
| (a) | (b) | (a) | (b) | |
| Two assumptions are made: that the preventive intervention has 50% (a) or 100% (b) efficacy (see text). | ||||
| Relative risk for gene | 1.0 | 1.0 | 11 | 11 |
| Cumulative risk of premalignant lesions | 10% | 10% | 100% | 100% |
| Risk reduction % | 50% | 100% | 50% | 100% |
| Cumulative risk after intervention | 5% | 0 | 50% | 0 |
| Absolute risk reduction | 5% | 10% | 50% | 100% |
| NNT | 20 | 10 | 2 | 1 |
| Carrier frequency | 80% | 80% | 20% | 20% |
| NNS | 25 | 12.5 | 10 | 5 |
| (a) | (b) | |||
| NNT in the absence of screening | 16.4 | 8.2 | ||