Table 3 Sample sizes for hypothetical randomised controlled trials with different analysis plans/outcome variable
Predictors included in RCT analysis | Outcome variable at 1-year follow-upa | Sample size (n per group) | Total sample size | Allowance for 20% dropout |
|---|---|---|---|---|
Treatment group only (assumed difference 1 ml) | WMH volume | 8508 | 17,016 | 21,270 |
Treatment group + baseline WMH volume (R 2 = 0.88) | WMH volume | 987 | 1974 | 2468 |
Treatment group + age (R 2 = 0.19) | WMH volume | 6872 | 13,744 | 17,180 |
Treatment group + baseline Fazekas score (R 2 = 0.69) | WMH volume | 2628 | 5256 | 6570 |
Treatment group + all predictors used in multivariable analysis (R 2 = 0.90) | WMH volume | 821 | 1642 | 2054 |
Treatment group + all predictors used in multivariable analysis (R 2 = 0.90) and 2-year follow-up | WMH volume | 205 | 410 | 642b |
Treatment group only (assumed OR 1.2) | Fazekas score | 1499 | 2998 | 3748 |
Treatment group only (assumed OR 1.25, upper 95% CI) | Fazekas score | 1001 | 2002 | 2504 |
Treatment group only (assumed OR 1.16, lower 95% CI) | Fazekas score | 2262 | 4524 | 5656 |
Treatment group + baseline Fazekas (multiplierc 5.04) | Fazekas score | 7554 | 15,108 | 18,886 |
Treatment group + baseline hypertension (multiplierc 1.00057) | Fazekas score | 1500 | 3000 | 3750 |
Treatment group + baseline age (multiplierc 1.012) | Fazekas score | 1515 | 3030 | 3788 |
Treatment group only (median increase 0.5) | Fazekas score | 87,216 | 174,432 | 218,040 |