Sample size calculations for future study
The overall within-centre standard deviation (SD) of the primary outcome at baseline was 96 minutes (1.6 hours), and between centre SD was 44 minutes (0.7 hours). In addition, the mean value of the primary outcome at baseline was 359.6 minutes, and we recruited a mean of 15.6 participants providing valid primary outcome data per cluster. Therefore, using the equation (6) shown in Hayes and Bennett [In contrast, the subjective outcome measure of sedentary behaviour (the OSPAQ, measuring number of hours sitting) showed substantial variability in our study. The overall within-centre SD at baseline was 7.2 hours and between-centre SD was 2.3 for the pre-lockdown data. In the post-lockdown data, the between-centre SD was 5.2 hours with a between-participant SD of 6.8 hours.