Ross suggests a neat natural experiment that could potentially identify an exogenous effect of caddy on a golfer’s performance. The imposed exogeneity comes from the fact that for years all players in the Masters were required to use an Augusta National Club caddy, whereas in the other three major tournaments, golfers could use their own caddy. So one way of looking at the effect of caddying is to see how much predictive power player rankings at the other tournaments have for performance at the Master’s relative to how much predictive power they have for each other. If there had been no change of rule, such an exercise might be suggestive but hardly conclusive, as there might be something about the Master’s that encourages a different kind of player. (My little bit of golf knowledge suggests that this might not be a huge deal–the major tournament that is the most different from the other three is the British Open.)
But the rule did change in 1982, after which golfers could use their own caddy at the Master’s. So here is my suggested test. Take the 5 years prior to the rule change and the 5 years after. For each major tournament in those 5 years, find the subset of players who played in that tournament and each of the previous three (i.e. all 4 majors over a 12-month period), and note the ranking out of that subset of players in each tournament. Then for every pair of players the player with the highest average ranking in the three previous tournaments also had a higher ranking in the 4th. If caddying is important, we would expect to see that the fraction of pairs where the pairwise ranking stayed the same was lower for the pre-1982 Master’s than for the other three tournaments (due to the effect of randomly assigned caddies), but that that difference disappeared after 1982.
I don’t have time to do this, so I put it out there if someone wants to jump in as a co-author and do the legwork. My guess is that golf is such a high-variance game that there simply won’t be enough data to tease out any statistically significant result, but it might be interesting. Or maybe it could be an Honours project.