Sample size calculation. The study was designed to detect a difference of 1 point in pain intensity measured with the Pain Numerical Rating Scale 20 (estimate for standard deviation1.84 points)31 and a difference of 4 points in disability associated with low back pain measured
with the Roland-Morris Disability Questionnaire 21,22 (estimate for standard deviation4.9 points).31 The following specifications were considered: statistical power of 80%, alpha level of 5%, and follow-up loss of 15%. Therefore, the study will require a sample of 74 patients per group (148 in total).