Is it fair to compare BPR ratings across different positions? For example, let's say player X is an average starting pitcher with a PBR of 4.5 and player Y is a top line relief pitcher with an PBR of 3. Would it be bad to trade X for Y because of the difference in the PBR's? What I'm trying to say is, is the PBR the only variable we should use when trying to make a trade? Thank You