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 players Index will be determined by the standard deviation of the distribution of his scoring differentials and the Bonus for Excellence (BFE) – currently .96.

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.   Therefore, a players Handicap Index should be reduced by .15∙σ multiplied by the BFE.   If a player’s standard deviation is 3.5, the reduction in Handicap Index would be 0.5 ( i.e., .15∙3.5∙.96) .  The impact on a player’s Course Handicap would be would be none or one stroke.  Given all of the other complexities of the WHS (Daily Handicap Index, Daily Course Rating based on weather conditions,  limits on Handicap Index reductions,  possible reduction of the BFE, 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.  A typical player’s reaction to all of this might be “Whatever.”



[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)


3 comments:

  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.

    ReplyDelete
  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.

    ReplyDelete
  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."

    ReplyDelete