Sample size and statistical analyses
pain
REGRESSION, SENSITIVITY
Assuming attrition of 25% from those (337) participants still enrolled in the trial at 12 months after randomisation, a sample size of 253 for the longer-term follow-up would have 80–90% power to detect differences in the range 0.94–1.09 points on the BPI severity scale, using the observed standard deviation (SD) of 2.5 and a two-sided 5% significance level. With this SD, these detectable differences are equivalent to 0.38–0.43 SDs. The 2:1 intervention:control randomisation ratio means the projected sample sizes were 169:84.All analyses were conducted in Stata version 17.0. Following an update to the STAR CONSORT participant flow diagram and descriptive statistics exploring any additional attrition, the primary analysis for this study was a comparison of the two treatment groups (as randomised) according to the co-primary outcomes, pain severity and pain interference as measured by the BPI. As well as descriptive statistics and plots, this involved linear regression models adjusting for baseline values of the outcomes and minimisation/stratification variables as fixed effects, presenting estimates, confidence intervals and p-values.Sensitivity analyses considered a multiple imputation chained estimation (Stata command mi impute chained) based on missing at random assumption, and further analyses making the most extreme assumptions about missing data across the two arms (that is, in the first case, assuming the worst possible outcome for the intervention arm, and the best for the control arm, and in the second case, assuming the best possible outcome for the control arm, and the worst possible outcome for the intervention arm.Similarities and differences at trial baseline between participants who provided four year follow-up data and those who did not were examined.