Normality analysis of numeric rating scale scores in patients with chronic axial spine pain before and after medial branch blocks: a multicenter study.

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The statistical analysis typically used to compare pain before and after interventions assumes that scores are normally distributed. The present study evaluates whether numeric rating scale (NRS) scores, specifically NRS-11 scores, are indeed normally distributed in a clinically relevant cohort of adults with chronic axial spine pain before and after analgesic intervention. Retrospective review from 4 academic medical centers of prospectively collected data from a uniform pain diary administered to consecutive patients after they had undergone medial branch blocks. The pain diary assessed NRS-11 scores immediately before injection and at 12 different time points after injection up to 48 hours. D'Agostino-Pearson tests were used to test normality at all time points. One hundred fifty pain diaries were reviewed, and despite normally distributed pre-injection NRS-11 scores (K2 = 0.655, P = .72), all post-injection NRS-11 data were not normally distributed (K2 = 9.70- 17.62, P = .0001-.008). Although the results of parametric analyses of NRS-11 scores are commonly reported in pain research, some properties of the NRS-11 do not satisfy the assumptions required for these analyses. The data demonstrate non-normal distributions in post-intervention NRS-11 scores, thereby violating a key requisite for parametric analysis. We urge pain researchers to consider appropriate statistical analysis and reporting for non-normally distributed NRS-11 scores to ensure accurate interpretation and communication of these data. Practicing pain physicians should similarly recognize that parametric post-intervention pain score statistics might not accurately describe the data and should expect articles to utilize measures of normality to justify the selected statistical methods.