6Levin, A. Lin, C. and Chu,C. Unit Root Tests in Panel Data:As- ymptotic and Finite Sample Properties [ J ]. Oxford Bulletin of E- conomics and Statistics ,2002,108 : 1-24.
7Breitung,J. The Local Power of Some Unit Root Tests for Panel Data[ J]. Advances in Economics,2000 (15) : 161-177.
8Im, K. S, M. H. Pesaran, Y. Shin. Testing for Unit Roots in Het- erogeneous Panel Data[J]. Journal of Economics, 2003 (109) : 53-74.
9Maddala, G. S. , Wu, S. A. Comparative Study of Unit Root Tests with Panel Data and a new simple test[J]. Oxford Bulletin of E- conomics and Statistics, 1999 (108) : 1-24.
10Hardi, K. S. Testing for stationarity in heterogeneous panel data [ J ]. Econometrics Journal,2000 (3) : 148-161.