This book is dedicated to the memory of Gis?le Mersch whose life ended prematurely in June 2002. Back in the 1970s, when few people were using them, Gis?le introduced me to the arcane secrets of then advanced m- tivariate statistical methodologies. I was already involved in more classical statistical studies undertaken at Paris Observatory with Jean Jung: developing and applying maxim- likelihood algorithms to stellar photometric and kinematic data...