Sample size
Sample size calculation was based on number of sickness absence days (primary outcome). Assuming an average of 60 sickness absence days per year for the control group and 50 days for the intervention group [alpha 0.05, standard deviation (SD) 30], we would need 149 participants in each arm for a Wilcoxon rank-sum test with 80% power. As power calculations are heavily based on assumptions, the planned sample size was increased to 250 in each group. The sample size calculation was performed by a statistician outside the project group.