Dear Dr. Bates, Thank you very much for your response. I had consulted the algorithm described in Pinheiro and Bates. However, what I don't understand (among other things) is why my two parameters appear to be estimated at different grouping levels (based on the DF values). Affect this different values of DF at the estimates parameters? The estimates fixed effects were get at the same level of grouping? I apreciate any response. Lic. Gabriela Escati Pe??aloza Biolog??a y Manejo de Recursos Acu??ticos Centro Nacional Patag??nico(CENPAT). CONICET Bvd. Brown s/n??. (U9120ACV)Pto. Madryn Chubut Argentina Tel: 54-2965/451301/451024/451375/45401 (Int:277) Fax: 54-29657451543 __________________________________________________ Correo Yahoo! Espacio para todos tus mensajes, antivirus y antispam ??gratis! ??Abr?? tu cuenta ya! - http://correo.yahoo.com.ar