I'm writing program in which I generate pseudo random expressions with the hope of finding one that closely approximates the given data set. The functions map from R^m (an m-tuple of reals) to a real. What I need is a way to rate the functions by their accuracy. Are there any known methods for doing this? Maybe something from stats. Ideally this would be a method that would take a list of expected outputs and actual outputs and return a real number. A uniform distribution would be good, but not required.