This is the fourth version of a model that five years ago we set out to build and estimate along the lines of the continuous time approach clarified In chapter 1. Previous versions appeared in journal articles and conference proceedings, where the space is notoriously limited. Therefore we welcome the possibility of publishing a book-length treatment of this fourth version, so that we can describe its theoretical and empirical aspects in some detail. Although we have worked closely together and accept joint responsibility for the whole book, chs. 1 and 2 and appendix I have been written by G....