Abstract
The hierarchical linear model (HLM) has become popular in behavioral research, and has been widely used in various educational studies in recent years. Violations of model assumptions can have significant impact on the model estimates. The purpose of this study is to conduct a sensitivity analysis of two-level HLM by exploring the influence of outliers on parameter estimates of HLM under normality assumptions. A simulation study is performed to examine the bias of parameter estimates with different numbers and magnitudes of outliers given different sample sizes. Results indicated that the bias of parameter estimates increased with the magnitudes and number of outliers. The estimates have bias with a few outliers. A robust method Huber sandwich estimator corrected the standard errors efficiently when there was a large proportion of outliers.
