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Conference Paper/Presentation

Publication Date



Andrew D. Vigotsky; Northwestern University

A. Vania Apkarian, PhD; Northwestern University


Biology | Mathematics | Statistics and Probability


Previous research suggests greater baseline variability is associated with greater pain relief in those who receive a placebo. However, studies that evidence this association do not control for confounding effects (natural history and regression-to-the-mean); for this reason, we analyzed data from two randomized clinical trials (Placebo I and Placebo II, N = 134) while adjusting for confounding effects via a no-treatment group. Results agree between the two placebo groups: both placebo groups showed a negligible correlation between baseline variability and adjusted response (r sp (CI 95% ) = 0.13 (−0.09, 0.37) and 0.01 (−0.15, 0.20) for Placebo I and II, respectively). Drug groups also showed similar, weak correlations (rsp = −0.16–0.08; max CI 95% = −0.39–0.31). When modeled as a linear covariate, variability only accounted for an additional 1% of the variance in post-intervention pain across both studies; the inability of variability to account for substantial variance in pain response highlights that previous results concerning variability and treatment response may be inconsistent. Indeed, the relationship appears to be neither consistently specific nor sensitive to improvements in the placebo group. Researchers and clinicians should not rely on using baseline pain variability as a prognostic factor for improvement following placebo.



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