Statistical analysis
REGRESSION, EVENTS, SECONDARY
All statistical analyses were based upon the randomised groups irrespective of compliance with allocation at the cluster (site) level. They were carried out using Stata version 15. Overall study and intervention group level data were summarised using a number of events and percentage for binary variables, mean and standard deviations (SDs) or median and interquartile range (IQR) or ranges at site and participant level where appropriate.The primary outcome was analysed as the proportion of returned diaries at the site level. Secondary outcomes were average number of completed pages, average time to diary return (days) and the average total cost (£) also at the site level. For all secondary outcomes, analyses were based also upon sites where at least one diary was returned. Whether the symptom diary was returned, and if it was, when it was received and how many pages were completed was available for all participants. Diaries were due to be returned at the earliest on day 15 after randomisation. A completed page was one where all possible data had been entered by the participant. Where a participant had no more symptoms they could indicate this at the top of the page and not answer any more questions, this would also be considered a complete page. There was no imputation of outcome data for sites where no one was recruited, or where a participant did not return their symptom diary.Due to the unanticipated small number of available pairs with both sites with outcome data, the main analysis of the primary and secondary outcomes was carried out using linear regression to estimate the mean difference in the proportion between the intervention groups. The intervention group was the sole explanatory variable. The linear regression was carried out in Stata using the cluster option to implement cluster robust variance estimation to account for site pair (a paired comparison had originally been planned). A sensitivity analysis for the primary and secondary outcomes used the same regression model but weighted the observations according to the number recruited or number of diaries returned depending upon the outcome. Other sensitivity analyses were also carried out for all the outcomes. These were a Mann–Whitney 