hen <code>HLMdiag</code> was created in 2014, it made diagnostic procedures available for HLMs that had not yet been implemented in statistical software. Over the past 6 years, other packages have gradually implemented some of these procedures; however, diagnosing a model still often requires multiple packages, each of which has its own syntax and quirks. <code>HLMdiag</code> provides diagnostic tools targeting all aspects and levels of hierarchical linear models in a single package. <code>HLMdiag</code> pr