## Tuesday, October 15, 2019

### How Will the World Handicap System Affect Your Index?

A player’s Handicap Index is based his adjusted scores and the Course and Slope Ratings.  The USGA Course Rating is based on a model which predicts the better half of scores of the 288 competitors in the U.S. Amateur Championships.  The Bogey Rating is equivalent to the average of the better half of a bogey golfer’s scores under normal playing conditions.[1]  Under the World Handicap System (WHS) that will become effective January 1, 2020, however, a player’s Handicap Index will be based on the better 40 percent of a player’s differentials (i.e., best 8 of 20).  The size of any reduction in a player’s Handicap Index depends on whether the Course and Slope Ratings are changed to reflect the drop in the number of best scores used—i.e., the current Course Rating is an estimate of the performance of a scratch handicap when his best 10 scores are used and not his best 8 scores.

It is unlikely the Scratch and Slope Ratings will be changed to reflect the changes under the WHS.   The cost of changing USGA models and the Course and Slope Ratings at courses is prohibitive.   So what will be the effect on a player’s Index given the Course and Slope Ratings stay the same? Any change in the Slope Rating due to the WHS is negligible and is neglected here.[2]  The change in a player's Index will be determined by the standard deviation of the distribution of his scoring differentials

Assuming a normal distribution, the average of a player’s ten best differentials is .8∙σ below his mean differential. The average a player’s 8 best differentials is .95 below his mean differential.   A player's Handicap Index  under the current system is:

Current Index = (Mean Differential - .8∙σ)∙.96 (.96 is termed the Bonus for Excellence)

Under the WHS, the Bonus for Excellence is eliminated.   The player's WHS Handicap Index is:

WHS Handicap Index = Mean Differential -.95∙σ

The change in the player's Index when the WHS is implemented is:

WHS Index Change = WHS Index - Current Index  =  .04 Mean Differential -.18∙σ

The change in Handicap Index will depend upon a player's mean differential and standard deviation.  If a player had a mean differential of 10 and a standard deviation of 3.5, his Handicap Index would decrease by 0.23  As shown in the Table below, the impact on a player's Handicap Index would be small or negligible.  Given all of the other complexities of the WHS (Daily Handicap Index, Playing Condition Calculation based on weather conditions,  limits on Handicap Index reductions,  and changes in Equitable Stroke Control) a player may not notice a change in his Index nor understand the cause for the change if he did notice.

Estimated Change in Handicap Index Under the World Handicap System

 Mean Differential Standard Deviation Change in Index 0 3.0 -0.54 10 3.5 -0.23 20 4.0 -0.01 30 4.5 +0.39

A typical player’s reaction to all of this might be “Whatever.”  There are other changes in the offing--e.g., playing handicap, reduction for exceptional performance. These changes will be examined in future posts.

[1] Stroud, R.G., Riccio L.J.,” Mathematical Underpinnings of the slope handicap system,” Science and Golf, E & FN Spon,  London, 1990, pp. 141-146.
[2]  The decrease in the Course Rating based on 8 out of 20 differentials is likely to be small.  To estimate the revised Course Rating assume a player’s differentials are normally distributed with a standard deviation of σ.   The average of a player’s ten best differentials will be approximately .8∙σ below his mean differential.  The average of a player’s eight best differentials will be approximately .95∙σ below his mean differential.  So to keep a “scratch player” a “scratch player” the Course Rating should drop by .15 ∙σ.   Assume the distribution of a scratch player’s differential has a standard deviation of 2.5.  In that case, the new Course Rating should be 0.4 strokes (.375 strokes rounded up)
Similarly, if the average Bogey player has a standard deviation of 3.5 strokes, the Bogey Rating should be reduced by 0.5 strokes.
The men’s revised Slope Ratings will then be:

Revised Slope Rating = 5.381 ((Old Bogey Rating -0.5) – (Old Course Rating -0.4))
= Old Slope Rating – 5.381∙(-0.1)
= Old Slope Rating -1.0 (rounded up)

[2] The decrease in the Course Rating based on 8 out of 20 differentials is likely to be small.  To estimate the revised Course Rating assume a player’s differentials are normally distributed with a standard deviation of σ.   The average of a player’s ten best differentials will be approximately .8∙σ below his mean differential.  The average of a player’s eight best differentials will be approximately .95∙σ below his mean differential.  So to keep a “scratch player” a “scratch player” the Course Rating should drop by .15 ∙σ.   Assume the distribution of a scratch player’s differential has a standard deviation of 3.0.  In that case, the new Course Rating should be 0.5 strokes (.45 strokes rounded up)

Similarly, if the average Bogey player has a standard deviation of 4.0 strokes, the Bogey Rating should be reduced by 0.6 strokes.

The men’s revised Slope Ratings will then be:

Revised Slope Rating = 5.381 ∙ ((Old Bogey Rating -0.6) – (Old Course Rating -0.5))

= Old Slope Rating – 5.381∙(-0.1)

= Old Slope Rating -1.0 (rounded up)

1. I agree with your assessment of the Index and the indifference by championship or core 18-hole golfers. However I believe the greater impact will be the affect on the aging 9-hole golfer population as 8 of 20 18-hole scores translates to 16 of 40 9-hole scores. And, considering most golfers play more poorly as they age, if they generally post 10 or fewer 9-hole scores a year, then their WHS Index could be held hostage to scores from 3-4 years ago. Couple this with the fact that the USGA has eliminated the "N" (9-hole) Index.

2. "Held hostage" seems a little strong. The player you describe only plays nine holes about once a month. It seem unlikely that such a player is involved in high stakes games where a stroke or two difference in handicap can be crucial. In my imaginary world where sandbaggers have become extinct, our player is more concerned with comradery and fresh air than drubbing his good friends. If I have mistaken the character of our player, he is certainly able to play a few more rounds to make his Handicap Index a more accurate estimate of his potential.
Thanks for the comment and your concern for ageing players such as myself.

3. The state associations recently released the details for the new 2020 rules. I've already applied the new system changes to my league, (over 170 players, with ages ranging from upper 70s to late teens, and skill levels ranging from plus handicappers to extremely high handicappers). The early indications coincide with the comment already posted. We run a 9-hole handicap, basically meaning the new rule means we'll be using 16 of last 40 9-hole posts. Most casual players are posting 30-40 rounds a year, so they will be "held hostage" with a year's worth of data. (Note: we only use league data for handicap purposes.)

The two components of the new system having the biggest effect are the extended data base (40 scores) and the exceptional score trigger. When triggered, exceptional scores will now be applied to all 40 scores, meaning that the "safeguard" will be felt until that score cycles out of the 40 score databank. Playing off the 18-hole trigger of 7.0, this basically means the unstated 9-hole net differential of 3.5 is needed to trigger the adjustment.

The volatile players who can occasionally go low will be affected by the new rules, and with their newly calculated course handicap, they will experience some serious "sticker shock." As much as the USGA has downplayed the effect, it will be much stronger felt once applied. But rightfully so. Based on the data I've applied to date, the new system will more accurately represent a player's potential, as the new safeguards do tackle "sandbagging."

4. In areas where 9-hole leagues exist (in my state of Michigan we estimate there are more than 5000 leagues representing 250k golfers), the average course hosts a dozen leagues and each league averages 14-15 rounds a season (which usually lasts late April to late August). Effectively, league players play at minimum a 9 hole round once a week and many league players do not play outside of league play (or they just post their league rounds (above comment)). Consequently, yes, many of these aging core golfers may have their handicaps held hostage to older scores. This is a key technical reason why the WHS will not work well for leagues.

5. I should add that the WHS paints a broad brush; considering golf as a homogeneous unit. It isn't. The truth is that golf is a community sport and those communities are different. A public course 9 hole league is a world apart from a private club tournament series, and forcing a single handicap system on both will fail one or the other. Maybe that's why the USGA Handicap System has contracted over the past two decades rather than grown.

6. You mention above that only league scores count. I assume this means you keep track of handicaps outside of the MGA (GHIN) system. Tournament Committees have the authority to assign handicaps different from a player's USGA handicap. Some clubs have adopted the Knuth Point System which lowers a player's handicap if he/she has been a frequent winner. In some tournaments, a player has to play to his low Index of the year. Since apparently you have all of the data, you could stick with the best ten with no trigger. The limit to your authority is only the reaction of the players. If a handicap system is seen as unjust, some players will drop out. I doubt that would be the case, if you just stuck with the old system.
You are lucky to be in Michigan--except for this time of year--where public golf is accessible and relatively inexpensive.

7. Most golf leagues run their own handicapping system completely separate of any USGA calculation just because they are generally 9 hole leagues played once a week. I run a golf league website and the average # of rounds league use rounds out to 6 (6 9-hole rounds, to be exact), while dropping the highest 1 or 2.