I would also ask about the intercept value, and its implications in a good correlation... Thanks! S. -- *********************************** Forli Stefano Dip. Farmaco Chimico Tecnologico Universita' degli Studi di Siena Via Aldo Moro I-53100 Siena, Italy Phone: ++39 0577 234307 Fax: ++39 0577 234333 ***********************************