In the last decade, methods based on propensity scores (PSs) have been frequently used in multiple sclerosis (MS) studies comparing disease-modifying treatments in nonrandomized observational settings.1 Propensity score adjustment was applied even in situations when all the necessary conditions for its applicability were not satisfied. The PS adjustment can reduce the intrinsic selection bias of nonrandomized studies only if all the confounders are measurable and are at least minimally overlapped between the treatment groups. Sometimes the calendar period or the geographical region of study conduction can be completely nonoverlapping between the compared groups. In such cases, in the causal inference jargon, the positivity assumption, requiring that the probability to receive any of the treatments in all the PS strata is higher than 0, is violated. We will show, with a practical example, the extent of failure of PS adjustment when the positivity assumption is violated.
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Signori A, Pellegrini F, Bovis F, Carmisciano L, de Moor C, Sormani MP. Comparison of Placebos and Propensity Score Adjustment in Multiple Sclerosis Nonrandomized Studies. JAMA Neurol. 2020;77(7):902–903. doi:10.1001/jamaneurol.2020.0678
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