OBP is inherently more stable than AVG. It has nothing to do with sample size.

The regression model controls for batter quality, so the relevant information is there.

The problem with using the 3 year RISP variable is certain to occur, not unlikely. The problem is that 3 year RISP isn’t a pure measure of a player’s clutch ability (if such ability exists). Suppose Pujols is .420 overall and .400 with RISP, while Adam Everett is .300 and .320. Your regression doesn’t “know” that Pujol’s .400 is poor clutch performance, while Everett’s .320 is good clutch hitting. The difference between RISP and overall OBP, or something similar, is what you need.

This piece reminded me of a chapter in Baseball Between the Numbers by the Prospectus crew. The title was “Is David Ortiz really Mr. Clutch?”. Their initial principle was much the same as yours–if clutch hitting really exists, past performance should have predictive value. To test it, they used one of their situational win probability metrics (dubbed “clutch score”) and divided them into two groups: odd years and even years, encompassing a hitter’s entire career. They then generated a scatter plot with the “clutch score” in a hitter’s even years on the X axis and the “clutch score” in a hitter’s odd years on the Y axis. Their premise being if clutch hitting is a repeatable skill, a “clutch hitter” will have positive values in both their even years and odd years, leading to a positive correlation. This is that scatter plot. As you may have guessed, their conclusion–a conclusion shared by many who have studied the subject–was that if clutch hitting exists, it’s such a small piece of the puzzle that “it’s probably folly for a club to go looking for clutch hitters–the ability just isn’t important enough in the bigger scheme of things”.

The second problem (if I understand it) is unlikely to occur. In any event, the indicator I used is imperfect for many reasons, but if “clutch” was there it should show up.

Also, if you want to see if prior clutch hitting predicts future clutch hitting, 1989-91 RISP is not the variable you want. You need a measure of the hitters’ clutch performance relative to their other performance, such as RISP(89-91) minus Overall OBP(89-91). In your model, a hitter’s 1992 and prior clutch performance (defined as RISP minus Overall) could be identical, and yet your RISP(89-91) variable still won’t have much power because it mostly tells you the hitter’s overall ability, not his clutch performance relative to his overall performance. (Of course, even if you do this correctly you probably still won’t find that past clutch differential has much predictive power.)