Sample size {14}
syncope
The trial sample size has been informed by (i) the need to generate evidence sufficiently compelling to influence regulators and policy-makers; (ii) NHSBT's duty of care to 900,000 blood donors per year, making it vital for the service to evaluate even small changes in VVR rates; (iii) NHSBT’s objective to optimise blood collection procedures, improve donor return rates, and improve donor (and staff) well-being and satisfaction; and (iv) the need for appropriate power to study determinants of VVRs, both singly and in combination.Power calculations have been based on the primary endpoint (defined above), assuming a 5% type I error probability and service-wide cluster randomisation (involving 73 teams with ~15,000 whole blood donations per 9-month period per team, and overall primary outcome rate of 0.18% per donation). For the two interventions being assessed using a cross-over design (i.e. isotonic drink and time on donation chair after donation), there is >90% power to detect an odds reduction of >8% (odds ratio of 0.92). For the two interventions being assessed using a stepped-wedge design (i.e. modified AMT and psychosocial intervention), there is >90% power to detect an odds reduction of >13% (odds ratio of 0.87) (Fig. Statistical power of the trial. Calculated for the two interventions in the study were assessed using a cross-over design (i.e. isotonic hydration and time on donation chair after donation; red line) and the two interventions in the study were assessed using a stepped-wedge design (i.e. modified AMT (applied muscle tension) and psychosocial intervention; blue line). Each intervention was tested at a Bonferroni corrected level (type I error) of 0.0125. The study design has four possible sequences for interventions assessed using a stepped-wedge design (X-I-I-I, X-X-I-I, X-X-X-I, X-X-X-X), and two possible sequences for interventions assessed using a cross-over design (X-I-X-I, I-X-I-X) with a study duration of 3 years and each period lasting 9 months (X = current practice, I = intervention). Power calculations assume 15,000 whole blood donations per 9-month period in each donation centre, with an overall rate of severe (syncope) VVRs of 0.18% per donation and a between-centre variance in log odds of 0.06 (estimated from 2014/15 NHSBT data)