The existence of robust and gracile forms of the Triassic theropod dinosaur Coelophysis and some other ceratosaurs is well documented, but the forms themselves are poorly quantified. Allometric and other bone-to-bone comparative methods used by previous workers are instructive, but only yield interpretation-dependent results of the overall character of the two forms. We use probability plots to compare biometric data for Coelophysis bauri to a standard equation (the Normal probability density function). This produces an unambiguous definition of the robust and gracile forms in terms of the mean sizes and standard deviations of particular bones or bone series. Most of the data we use are from E.H. Colbert's (1989: Mus. N. Az. Bull. 57) published measurements and included some juveniles. Within the accuracy of the analysis, (no better that~15% due to sample size), the robust and gracile forms each constitute 50% of the population, strongly supporting sexual dimorphism as an explanation of the forms.
Some preliminary samples from the analysis include (all dimensions in mm): robust form; skull length (mean/standard deviation) 185/60, femur length 170/50, 3rd metatarsal length 120/12, dorsal series length 350/400/?, gracile form; skull length 90/20, femur length 120/8, 3rd metatarsal length 80/5, dorsal series length 250-300/?.
Clearly, more data are required to increase the confidence level of this analysis, but we show a statistical means to quantify the two forms. We solicit measurements from those in possession of Coelophysis material in order to increase the sample size.