Flexible distributional models for meta-analysis of reading fluency outcomes from single-case designs: An examination...

There is growing interest in statistical modeling of data from single-case design (SCD) research. However, currently available methods such as hierarchical linear models and generalized linear mixed models have assumptions that may limit their utility for applied SCDs, such as those that use curriculum-based measures of academic performance as outcomes. In the present paper, we demonstrate use of a flexible class of distributional models, known as generalized additive models for location, scale, and shape (GAMLSS), to evaluate different distributional families and modeling specifications for reading curriculum-based measures of reading fluency data drawn from SCD studies of academic interventions. Using Bayesian methods and graphical posterior predictive checks, we evaluated GAMLSS based on normal (Gaussian), Poisson, and negative binomial distributional families. We also evaluated the extent to which the dispersion, or variability of outcomes, itself varied across studies and across participants within studies. We found that negative binomial models with heterogeneous dispersions fit better than other distributional families and closely reproduced features of the observed data. Findings highlight the need to consider a broader set of distributional families when developing meta-analytic models of SCD data as well as the need to consider how the degree of dispersion may vary from study to study. We discuss implications for future methodological research and for meta-analysis of SCDs.