Mixed effects models have become a critical tool in all areas of psychology and allied fields. This is due to their ability to account for multiple random factors, and their ability to handle proportional data in repeated measures designs. While substantial research has addressed how to structure fixed effects in such models there is less understanding of appropriate random effects structures. Recent work with linear models suggests the choice of random effects structures affects Type I error

more »

... such models (Barr, Levy, Scheepers, & Tily, 2013; Matuschek, Kliegl, Vasishth, Baayen, & Bates, 2017). This has not been examined for between subject effects, which are crucial for many areas of psychology, nor has this been examined in logistic models. Moreover, mixed models expose a number of researcher degrees of freedom: the decision to aggregate data or not, the manner in which degrees of freedom are computed, and what do when models do not converge. However, the implications of these choices for power and Type I error are not known. To address these issues, we conducted a series of Monte Carlo simulations which examined linear and logistic models in a mixed design with crossed random effects. These suggest that a consideration of the entire space of possible models using simple information criteria such as AIC leads to optimal power while holding Type I error constant. They also suggest data aggregation and the d.f, computation have minimal effects on Type I Error and Power, and validate appropriate approaches for dealing with non-convergence.