103 The short-term effectiveness of e-Exercise low back pain – a cluster RCT baseline that changed the regression coefficient for the between-group estimate by ≥10%. Potential interaction terms were explored. In the case of a statistically significant interaction term, stratified LMM analyses, controlling for the same variables as the primary analysis, were performed for the effect modifier. Sample Size The power calculation was based on the recommendations of Campbell et al (45) for cluster randomized trials and performed for the physical functioning primary outcome at the primary end point of the e-Exercise LBP study (i.e., 12-month follow-up). In addition, repeated measures of the primary outcome during follow-up were taken into account (46). An intraclass correlation coefficient of 0.05 was assumed. In addition, to detect a clinically relevant difference between groups at the 12-month follow-up, a difference of >6 points in physical functioning (ODI) (47,48), and an SD of 14.5 (49) were used in the sample size calculation. For the repeated measures of physical functioning, a correlation of 0.5 was estimated between baseline and follow-up measurements until the 12-month follow-up (46). On the basis of these assumptions (power 80%; α=.05) and an average cluster size of 5, a total of 165 patients were needed. With an expected dropout rate of 20%, a total of 208 participating patients (n=104 per arm) were needed. RESULTS Flow of Participants, Therapists, and Centers Through the Study From June 2018 to December 2019, 434 eligible patients with LBP were asked to participate in 58 physiotherapy practices. In 22 physiotherapy practices allocated to stratified blended physiotherapy and 20 practices allocated to face-to-face physiotherapy, 47.9% (208/434) patients were included (Figure 1). 5
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