An investigation into in-sample and out-of-sample model selection for nonstationary autoregressive models.

The stationary autoregressive model forms an important base of time-series analysis in today's psychology research. Diverse nonstationary extensions of this model are developed to capture different types of changing temporal dynamics. However, researchers do not always have a solid theoretical base to rely on for deciding a-priori which of these nonstationary models is the most appropriate for a given time-series. In this case, correct model selection becomes a crucial step to ensure an accurate understanding of the temporal dynamics. This study consists of two main parts. First, with a simulation study, we investigated the performance of in-sample (information criteria) and out-of-sample (cross-validation, out-of-sample prediction) model selection techniques in identifying six different univariate nonstationary processes. We found that the Bayesian information criteria (BIC) has an overall optimal performance whereas other techniques' performance depends largely on the time-series' length. Then, we re-analysed a 239-day-long time-series of positive and negative affect to illustrate the model selection process. Combining the simulation results and practical considerations from the empirical analysis, we argue that model selection for nonstationary time-series should not completely rely on data-driven approaches. Instead, more theory-driven approaches where researchers actively integrate their qualitative understanding will inform the data-driven approaches.