Exploratory Analysis of Longitudinal Data: Quality Audits, Visualisation, and Pre-Modelling Decisions

This chapter equips readers with a reproducible EDA checklist for long-format panels, which includes enumerating persons and waves, auditing for missingness, duplication, consistency, validity, accuracy, and timeliness, and visualising distributions and trajectories. Using SAS (PROC FREQ, SORT, COMPARE, FORMAT/SQL, MEANS, SGPLOT, FACTOR), this chapter operationalises diagnostics into heat maps, histograms with kernel density, cohort trend plots, and contingency tables with Chi-Squared tests, and contrasts within- versus between-subject variation. “Case Study 1”’ anchors the workflow, including PCA as an option. Findings are translated into hypotheses and pre-modelling choices (e.g., transformations, trimming, interactions) that prepare fixed-effects specifications, while producing an auditable quality record for inference.