Sorry to take everyone's time. I found a negative value that was causing the problem. Chris Wilcox Dr. Chris Wilcox Research Fellow Department of Zoology and Entomology University of Queensland St. Lucia 4072 QLD Australia email: c.wilcox at uq.edu.au tel: 61.7.3365.1686 fax: 61.7.3365.1655 website: www.uq.edu.au/~uqcwilco "In God we trust all others must bring data" W. Edwards Deming