Methodological Considerations
SECONDARY, INTERACTIONS, RECRUITMENT
The power calculation, based on self-reported sick leave from the previous project, estimated that a sample of 182 employees was needed. A limitation is that the sample size calculation did not include estimation of design effect, which resulted in an under-powered study. The recruitment resulted in 191 employees, however 145 (76%) gave consent to social insurance data and 147 filled out the baseline assessment, which further reduced power for primary and secondary outcomes. Consequently, sample size was also low in analyses of per-protocol participants and matched participants, and sample size did not allow for sub-group analyses for individuals at risk, types of workplace nor for Covid-19 pandemic.The number of non-responders among employees was high at all time-points of data assessment, with a retention rate of 52.4% from recruitment to 6-months follow-up. It should be noted these figures are much higher than those reported in the previous, similar study [Conducting the study during the Covid-19 pandemic was challenging, and ideally, the program should be re-run during ordinary conditions. The pandemic affected recruitment, participation and delivery of intervention, which might have affected integrity and effectiveness. The pandemic might also potentially had different impact for different individuals and workplaces. Interactions on outcome variables was found for participation during the pandemic, but also for type of workplace, and they were both included as co-variates in analyses.Another methodological limitation was differences between employees in the ECO and the PE group, which was handled by complementary analyses on matched participants. The unbalanced groups seem to be associated with the different distribution of types of workplace between conditions, and hence might be related to the cluster aspects of the randomization and recruitment process. In future projects, the cluster design could be better handled with a larger power or a stratification of types of workplaces. An alternative analytic approach would be to use multilevel analyses, adding the cluster level in the analyses. In summary, for future similar projects, we recommend larger sample sizes, longer time for follow-up, effort to include a more diverse sample regarding gender, and actions to increase study participation and response rate.