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An empirical unravelling of Lord’s Paradox.

Xiao, Z. and Higgins, S. and Kasim, A. (2019) 'An empirical unravelling of Lord’s Paradox.', Journal of experimental education., 87 (1). pp. 17-32.


Lord's Paradox occurs when a continuous covariate is statistically controlled for and the relationship between a continuous outcome and group status indicator changes in both magnitude and direction. This phenomenon poses a challenge to the notion of evidence-based policy, where data are supposed to be self-evident. We examined 50 effect size estimates from 34 large-scale educational interventions and found that impact estimates are affected in magnitude, with or without reversal in sign, when there is substantial baseline imbalance. We also demonstrated that multilevel modeling can ameliorate the divergence in sign and/or magnitude of effect estimation, which, together with project specific knowledge, promises to help those who are presented with conflicting or confusing evidence in decision-making.

Item Type:Article
Keywords:Lord's Paradox, RCT, Multilevel Modelling, Evidence-Based Policy, Evaluation.
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(AM) Accepted Manuscript
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Publisher statement:This is an Accepted Manuscript of an article published by Taylor & Francis in Journal of Experimental Education on 07 Nov 2017, available online:
Date accepted:06 September 2017
Date deposited:29 August 2017
Date of first online publication:07 November 2017
Date first made open access:07 May 2019

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