This work was created as part of my role as a research associate at the Chair of Empirical Social and Economic Research at the University of Cologne. It was accepted as a dissertation by the Faculty of Economic and Social Sciences at the University of Cologne in the winter semester of 2007/08. I would like to express my heartfelt gratitude to my colleagues, friends, and family for their support over the past years. First and foremost, I thank my doctoral supervisor, Prof. Hans-Jürgen Andress, who inspired me to tackle this topic and supported me in many ways. I also benefited from the helpful comments and discussions with my former colleagues at the chair. I am especially grateful to Rebecca Lange for her quick and reliable proofreading of the first manuscript. I also thank my partner for his support.