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Publication - Professor Kelvyn Jones

    Understanding and misunderstanding group mean centering

    a commentary on Kelley et al.’s dangerous practice


    Bell, A, Jones, K & Fairbrother, M, 2018, ‘Understanding and misunderstanding group mean centering: a commentary on Kelley et al.’s dangerous practice’. Quality and Quantity, vol 52., pp. 2031-2036


    Kelley et al. argue that group-mean-centering covariates in multilevel models is dangerous, since—they claim—it generates results that are biased and misleading. We argue instead that what is dangerous is Kelley et al.’s unjustified assault on a simple statistical procedure that is enormously helpful, if not vital, in analyses of multilevel data. Kelley et al.’s arguments appear to be based on a faulty algebraic operation, and on a simplistic argument that parameter estimates from models with mean-centered covariates must be wrong merely because they are different than those from models with uncentered covariates. They also fail to explain why researchers should dispense with mean-centering when it is central to the estimation of fixed effects models—a common alternative approach to the analysis of clustered data, albeit one increasingly incorporated within a random effects framework. Group-mean-centering is, in short, no more dangerous than any other statistical procedure, and should remain a normal part of multilevel data analyses where it can be judiciously employed to good effect.

    Full details in the University publications repository