7Hausmana, J. and G. Kuersteiner,2008. "Difference in Difference Meets Generalized Least Squares:Higher Order Properties of Hypotheses Tests. " Journal of Econometrics. 144:371 - 91.
8Keen, M. and M. Marchand, 1997. "Fiscal Competition and the Pattern of Public Spending. " Journal of Public Economics. 66( 1 ) :33 - 53.
9Jerry Hausman and Guido Kuersteiner. Difference in Difference Meets Generalized Least Squares: Higher Order Prop- erties of Hypotheses Tests. Journal of Econometrics, 144:371-391, 2008.