Development of a method for comparing water yields from wells drilled in crystalline rocks as the basis of a probability study of well yields is reported. The method utilizes curves drawn from data collected during short-term pumping tests and is designed for use in crystalline rocks and in the absence of observation wells. All measurements are made on the pumped well. The method was developed to take into account the nature of ground water occurrence in the crystalline rocks of North Carolina piedmont, and in particular the Raleigh, North Carolina area. The curves and the principles on which they are based permit prediction of probable yields from a given well at a variety of pumping rates. (Author)