We estimated the model using longitudinal pooled panel data. Pooled panel data are repeated observations on a unit of analysis over several time periods. Given that the number of Virginia hospitals is relatively small, pooled panel data increase the precision of estimates through an increase in the number of observations (Cameron & Trivedi, 2005). However, models with pooled panel data rely on certain strict assumptions to achieve the efficiency of the results. Specifically, they assume that the variances of the error components are homoskedastic and exhibit no correlation over time. If these assumptions do not hold, estimated standard errors will be biased although the coefficient estimators are consistent (Greene, 2003). We assessed whether these assumptions were violated in our data to determine if robust standard error estimation was needed.