Abstract:

With the development of data-collection technics and increasing complexity of study designs, nested data widely exists in psychological research. Linear mixed-effects models, unfortunately with an unreasonable hypothesis that the residual variances are homogenous, are generally used in nested data analysis. Meanwhile, Mixed-Effects Location-Scale Models (MELSM) has become more and more popular, because they can handle heterogenous residual variances and are able to add predictors for the two substructures (i.e., mean structure denoted as location model and variance structure denoted as scale model) in different levels. MELSM can avoid estimation bias due to inappropriate assumptions of homogenous variance and explore the relationship among traits and simultaneously investigate the inter- and intra-individual variability, as well as their explanatory variables. This study, aims at developing the methods of model construction and sample size planning for MELSM, using simulated studies and empirical studies. In detail, the main contents of this project are as follows. Study 1 focuses on comparing and selecting candidate models based on Bayesian fit indices to construct MELSM, taking into consideration the estimated method for complicated models. We propose that model selection for location model and scale model can be completed sequentially. Study 2 explores the method of sample size planning for MELSM, according to both power analysis (based on Monte Carlo simulation) and the accuracy in parameter estimation analysis (based on the credible interval of the posterior distribution). Adequate sample size is required for both the power and the accuracy in parameter estimation. Study 3 extends the sample size planning method for MELSM to better frame the considerations of uncertainty. By specifying the prior distribution of effect sizes, repeating sampling and selecting model based on the robust Bayesian fit index suggested by Study 1, three main sources of uncertainty can be well controlled: the uncertainty due to unknown population effect size, sampling variability and model approximation. With the simulated study results, we are able to provide reliable Bayesian fit indices for MELSM construction, and summary the process of sample size planning for MELSM in both determinate and uncertain situations. Moreover, Study 4 illustrates the application of MELSM in two empirical psychological studies and verifies the operability of the conclusions of the simulated studies in practice. The unique contribution of this paper is to further promote the methods of model construction and sample size planning for MELSM, as well as provide methodological foundation for researchers. In addition, we plan to integrate the functions above to develop a user-friendly R package for MELSM and provide a basis for promotion and application of MELSM, which help researchers make sample size planning, model construction and parameter estimation for MELSM easily, according to their specification. If these statistical models are widely implemented, the reproducibility and replicability of psychological studies will be enhanced finally.

Key words: nested data, mixed-effects location-scale models, model construction, sample size planning