102 Chapter 5 Self-management ability was assessed using the Dutch version of the short form Patient Activation Measure (37). A higher score (range 0-100) indicates a higher level of selfmanagement. Health-related quality of life was measured using the EuroQol-5D-5L (38). A higher score (range 0-100) indicates a higher health-related quality of life. Patient self-reported adherence to prescribed home exercises was measured using the Exercise Adherence Rating Scale (39). A higher score (range 0-24) indicates better adherence. Other Measures Physiotherapists were asked to complete a registration form about the number of face-to-face sessions and report the applied treatment modalities per session. Patient characteristics and relevant clinical variables were assessed as part of the baseline questionnaire. Data Analysis Overview Descriptive statistics were used to explore baseline comparability and describe patients’ general characteristics, the number of face-to-face physiotherapy sessions, and the treatment modalities. To investigate selective attrition, general characteristics and primary baseline variables of dropouts and nondropouts were compared. All analyses were performed according to the intention-to-treat principle. Missing value analyses were performed by assuming the missing at random assumption. Multiple imputation was applied using multivariate imputation by chained equations with predictive mean matching for missing data in all outcomes. A total of 36 imputed data sets were generated, corresponding to the highest missing value percentage (40). For all analyses, a 2-tailed significance level of P<.05 was considered statistically significant. Analyses of Effectiveness Linear mixed models (LMMs) with random effects to control for correlation within patients and physiotherapy practices (41) were used to determine the short-term effectiveness of stratified blended physiotherapy compared with face-to-face physiotherapy on primary and secondary outcome measures. Regression coefficients with 95% CIs signifying the differences between stratified blended physiotherapy and face-to-face physiotherapy were estimated. Analyses were adjusted for predefined confounders (e.g., age, gender, and duration of pain (42–44)) that changed the between-group estimate by ≥10%. In addition, analyses were also adjusted for variables with a substantial difference at
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