not sure if you saw my edit earlier Why FIP: Ultimately, we want to use statistics that allow us to isolate the performance of the player we are attempting to analyze. ERA or RA9 do a terrific job telling us how many runs were scored while the pitcher was on the mound, but they do not necessarily tell us how well the pitcher performed because the number of runs a pitcher allowed is also dependent on their defense, luck, and the order in which events happened (often called sequencing). FIP is an attempt to isolate the performance of the pitcher by using only those outcomes we know do not involve luck on balls in play or defense; strikeouts, walks, hit batters, and home runs allowed. Research has shown that pitchers have very little control on the outcome of balls in play, so while we care about how often a pitcher allows a ball to be put into play, whether a ground ball goes for a hit or is turned into an out is almost entirely out of their control. As a result, a statistic (FIP) that estimates their ERA based on their strikeouts, walks, hit batters, and home runs while assuming average luck on balls in play, defense, and sequencing is a better reflection of that pitcher’s performance over a given period of time. This is highly related to the reasons why we care so much about Batting Average on Balls in Play (BABIP), specifically the fact that pitchers have very little control over their BABIP allowed. So, for me, if looking at a pitcher's performance, FIP. If looking at pitchers performance along with things quite a bit of of his control (to include other players' performance), ERA.