Parameter estimates and standard errors were calculated using the generalized estimating equation method incorporated into SAS (Version 9, SAS Institute, Cary, NC) using state as the clustering unit (Arellano, 1987; Liang and Zeger, 1986). This approach accounts for intra-correlation of observations within clusters (states) in estimating standard errors, which is the promary consideration in standard error estimation when applying this quasi-experimental approach (Angrist and Pischke, 2008; Bertrand et al., 2004).
