OBJECTIVE: The presence of bimodal outcome distributions has been used as a justification for conducting responder analyses, in addition to, or in place of analyses of mean difference, in clinical trials and systematic reviews of interventions for pain. The aim of this study was to investigate the distribution of participants' pain outcomes for evidence of bimodal distribution.
STUDY DESIGN AND SETTING: We sourced data on participant outcomes from a convenience sample of 10 trials of non-surgical interventions (exercise, manual therapy, medication) for spinal pain. We assessed normality using the Shapiro-Wilk test. Where the Shapiro-Wilk test suggested non-normality we inspected distribution plots visually and attempted to classify them. To test whether responder analyses detected a meaningful number of additional patients experiencing substantial improvements we also calculated the risk difference and number needed to treat to benefit (NNTB).
RESULTS: We found no compelling evidence suggesting that outcomes were bimodally distributed for any of the intervention groups. Responder analysis would not meaningfully alter our interpretation of these data when compared to the mean between group difference.
CONCLUSION: Our findings suggest that bimodal distribution of outcomes should not be assumed in interventions for spinal pain and do not support the automatic prioritisation of responder analysis over the between group difference in the evaluation of treatment effectiveness for pain.
}, year = {2017}, journal = {J Pain}, issn = {1528-8447}, doi = {10.1016/j.jpain.2017.03.004}, language = {eng}, }