The USGA suggests that the best way to determine how difficult it is to score on a given course or set of tees is to use the bogey rating. Here's how they explain it:
This rating is the evaluation of the playing difficulty of a course for the bogey golfer. It is based on yardage, effective playing length and other obstacles to the extent that affect the scoring ability of the bogey golfer. To figure out this number, other than from looking at this database, the bogey golfer should take the Slope Rating®, divide it by the set factor (5.381 for men, and 4.24 for women) and add that to the Course Rating. The result is a target score for the bogey golfer, and is a truer yardstick of the challenge that lies ahead for the particular set of tees. Example: 96.3- which predicts the bogey golfer's average of his ten best (out of twenty) scores would be approximately 96.3 from this particular set of tees.So how can we use this to adjust the shot efficiency formula? The first step I came up with is to determine what I'm calling the course adjustment factor (CAF):
CAF = BOGEY RATING / STD BOGEY RATING
where:
BOGEY RATING = [SLOPE / 5.381] + COURSE RATING
STD BOGEY RATING = [113 / 5.381] + COURSE RATING
I use 113 as the "standard slope" as that is the USGA definition of an average difficulty course. From there I simply multiply the calculated shot efficiency by the course adjustment factor to yield the adjusted shot efficiency (ASE).
As an example, let's look at 3 rounds that have the same shot efficiency score to see how the adjustment impacts the rating. As it turns out, I shot 79 in all 3 of these rounds, making it an even more ideal comparison:
1. 4/29/09 at Butter Brook Golf Club (Black Tees, 6702 yards, 72.6 rating, 133 slope). I shot 79 with 30 putts, for a SE of 0.737. The CAF comes out to 1.046, so the ASE is 0.771. By the conventional metrics I hit 54% of fairways and 50% of GIR.
2. 8/2/09 at Brookmeadow Country Club (Gold Tees, 6585 yards, 71.7/123). Again shot 79, this time with 31 putts, again the SE being 0.737. The CAF for those tees is 1.017, so the ASE this time is 0.749. The standard stats: 29% fairways and 44% greens.
3. 3/6/11 at Nancy Lopez Legacy CC in the Villages (Black Tees, 6906 yards, 73.2/135). Shot 79 with 29 putts for (again) a SE of 0.737. Here the CAF is 1.057, for an ASE of 0.779. In that round I hit 50% of fairways and 33% of GIR.
Obviously these three rounds are all quite similar, but the intuitive difference that comes from the course difficulty is fairly represented by the subtle difference in the adjusted shot efficiency.
Here's an even better example: in my 2008 round at Bethpage Black, I shot 94 with 36 putts, for a SE of 0.687. The Black is largely considered one of (if not the) most difficult courses in the country, with a rating (at that time) of 76.6 and a slope of 144. It's a great course to empirically understand how the slope rating works; even though the course is incredibly long, it plays even longer because of elevated greens, thick punishing rough, and the maniacally placed bunkers -- hence a high slope rating. So when you apply the course adjustment factor (the highest of any course I've played at 1.123), the ASE jumps all the way up to 0.771. And that's fair -- there's no way you can say an 84 at Braintree Municipal (rated at 70/123) has an equal shot efficiency to 94 at Bethpage, but that's how the calculation works out. After applying the course adjustment, the difference in the 2 rounds is almost 10% (0.771 vs. 0.686).
On a final note, here are my top 5 rounds by adjusted shot efficiency over the past 3 years:
- Butter Brook (7/17/11): score 73 (+1), SE 0.926, ASE 0.969.
- Cane Garden (5/18/10): score 80 (+8), SE 0.868, ASE 0.925.
- Granite Links (7/29/10): score 76 (+5), SE 0.851, ASE 0.913.
- Red Tail (7/25/09): score 75 (+3), SE 0.851, ASE 0.911.
- Butter Brook (6/23/10): score 74 (+2), SE 0.860, ASE 0.900.
