29 Development of the Exercise Adherence Scale (EXAS) in patients with low back pain Construct validity Construct validity refers to the extent to which scores obtained with a given measurement instrument relate to scores obtained with other instruments in a manner that is consistent with theoretically derived hypotheses, assuming the measurement instrument validly measures the construct of interest (18). Currently, there is no gold standard for the measurement of adherence to HBE recommendations. Therefore, construct validity was determined by testing convergent and divergent validity using four theoretical hypotheses. Convergent validity is the degree to which a measure correlates with other measures to which it is similar (19). Discriminant (divergent) validity is the degree to which a measure does not correlate with (diverges from) measures that are dissimilar (19). The factor “Barriers” has been found to be the strongest indicator of nonadherence to HBE programs in Dutch patients with LBP (20). Lack of time to exercise and lack of motivation to exercise were among the barriers reported most frequently by patients who did not adhere to HBE recommendations and were chosen for hypothesis testing of convergent validity. Essery, Geraghty, Kirby, and Yardley (2017) reviewed the literature on predictors of adherence to home-based physical therapies and found results for highly varied samples (21). They found that associations between adherence and a variety of possible predictors of adherence ranged mostly from no association to approximately r = 0.50. Therefore, the associations between perceived barriers and adherence were expected to be moderate (r = 0.30 to r = 0.50). Pain and disability were reported as factors by both adherent and nonadherent patients. Therefore, both pain and disability were expected to be unrelated to adherence to HBE recommendations and were chosen to test hypotheses of divergent validity (20). Consequently, the correlations between adherence to HBE recommendations, pain, and disability were expected to be low (r = 0.00 to r = 0.30). The resulting hypotheses to be tested were as follows: 1) The association between lack of time to exercise and the EXAS is between r = 0.30 and r = 0.50; 2) The association between lack of motivation to exercise and the EXAS is between r = 0.30 and r = 0.50; 3) The association between pain and the EXAS is between r = 0.00 and r = 0.30; and 4) The association between disability and the EXAS is between r = 0.00 and r = 0.30. Participants and setting For the validity study, 16 physical therapy primary care practices with 42 physical therapists participated and agreed to recruit patients with LBP according to the following inclusion criteria: the first visit to a physical therapist for the current episode of LBP as the primary complaint, current episode of LBP lasting more than four weeks at the first visit 2
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