One measure of the size and complexity of information systems that is growing in acceptance and adoption is function points, a user-oriented, nonsource line of code metric of the systems development product. Previous research has documented the degree of reliability of function points as a metric. This research extends that work by (a) identifying the major sources of variation through a survey of current practice, and (b) estimating the magnitude of the effect of these sources of variation using detailed case study data from commercial systems. The results of this research show that a relatively small number of factors has the greatest potential for affecting reliability, and recommendations are made for using these results to improve the reliability of function point counting in organizations.>