I am confused about the meanings of these two goodness-of-fit descriptors. If I have two independent variables and I wish to test the validity of a proposed functional relation between them (say I have measured electric potentials and currents and wish to test if the data reflects a linear relationship), which should I use? What are the differences between these tests? What texts would you recommend that have full proofs of such arguments? Forgive me if this is a stupid question, but I have had difficulty in finding a text explaining the differences.