Etienne Lehmann CREST, ENSAE   Timbre J360 15 boulevard Gabriel Péri Malakoff Cedex, 92 245 France   etienne.lehmann@ensae.fr   http://www.crest.fr/pageperso/lehmann/lehmann.htm           JEL Areas: E2 : Macroeconomics: Consumption, Saving, Production, Employment, and Investment: General (includes Measurement and Data)