SIPS 2026 DC

Multilevel models (MLM) are commonly used for analysing intensive longitudinal (ILD) in clinical psychology. However, existing guidelines and reviews of current practices tend to focus on cross-sectional clusters (individuals within groups) rather than the decision points inherent to longitudinal designs (repeated measures within individuals). To provide an overview of current practices in longitudinal studies specifically, we evaluated 100 randomly selected papers (published 2020-2025) against key criteria. Results showed reporting practices varied greatly. For instance, only 30% of studies reported their level-1 covariance structure (a key design feature to accommodate serial dependency), and 21% provided rationales for their choices. These findings highlight that several researcher degrees of freedom remain undisclosed and underreported in clinical psychology ILD research. To aid researchers in practice, based on our review, we provide a pedagogical overview of longitudinal methods and specific recommendations to guide more appropriate modeling and transparent reporting of longitudinal MLM.