Background: Randomization procedure in randomized controlled trials (RCTs) permits an unbiased estimation of\ncausal effects. However, in clinical practice, differential compliance between arms may cause a strong violation of\nrandomization balance and biased treatment effect among those who comply. We evaluated the effect of the\nconsolidation phase on disease-free survival of patients with multiple myeloma in an RCT designed for another\npurpose, adjusting for potential selection bias due to different compliance to previous treatment phases.\nMethods: We computed two propensity scores (PS) to model two different selection processes: the first to\nundergo autologous stem cell transplantation, the second to begin consolidation therapy. Combined stabilized\ninverse probability treatment weights were then introduced in the Cox model to estimate the causal effect of\nconsolidation therapy miming an ad hoc RCT protocol.\nResults: We found that the effect of consolidation therapy was restricted to the first 18 months of the phase (HR: 0.40,\nrobust 95 % CI: 0.17-0.96), after which it disappeared.\nConclusions: PS-based methods could be a complementary approach within an RCT context to evaluate the\neffect of the last phase of a complex therapeutic strategy, adjusting for potential selection bias caused by\ndifferent compliance to the previous phases of the therapeutic scheme, in order to simulate an ad hoc\nrandomization procedure.
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