This paper addresses the issue of building a repeat sales index, based on factors. It offers a more general and robust model based on a Principal Components Analysis (PCA). It applies this methodology to the Paris residential market. It uses the CD-BIEN database that contains more than 220 000 repeat sales transactions for residential apartments in the Paris area covering the period 1973-2001 periods. The PCA index for the Paris and close surrounding area is estimated and its characteristics and robustness are analyzed depending on: estimation period, choice of observations, periodicity and reversibility.