An Empirical Unraveling of Lord's Paradox

ABSTRACT

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.

KEYWORDS:

• Evaluation
• evidence-based policy
• Lord's Paradox
• multilevel modelling
• RCT

Acknowledgments

We would like to thank those EEF evaluators, particularly Dr. Ben Styles of the National Foundation for Educational Research in England and Wales, who attended a workshop on this topic and provided us with constructive feedback.

Funding

This research was funded by a grant to Durham University from the Education Endowment Foundation.