3 IZA Discussion Papers
IZA Discussion Paper No. 4103
published in: Advances in Econometrics, 2009, 25, 433-469
IZA Discussion Paper No. 3344
published in: Economics Letters, 2008, 100 (3), 369-372
IZA Discussion Paper No. 2712
revised version published in: Applied Economics Letters, 2009, 16 (11), 1165 - 1169