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Golf Handicaps Decoded
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Each time you enter a score into MyScorecard.com, the computer calculates or re-calculates your handicap. You see a number that may or may not change - but what really is going on behind the scenes? The calculation process can be represented by the formula below:
The formula appears quite complex, but by breaking it down into individual parts we can shed some light on the calculation.
The First Part
The first part of the formula calculates a differential, which stands for "score adjusted for difficulty of the course." The differential uses the slope and rating numbers to adjust your scores and create a "course-independent" measure of your performance (i.e. make scores played at courses of various difficulty 'apples to apples' comparable).
To calculate a differential, you subtract the rating from your score, and then multiply by 113 and divide by the slope. This is represented by the following part of the formula:
The best ten of your last twenty (BTLT) differentials are then chosen to calculate your handicap. Note that these ten may not be the same as your ten lowest scores. Why choose your best ten (and not your average ten)? Your differentials, being dependent on many different factors, will tend to fall in a bell curve distribution. But, because you are much more likely to shoot ten strokes above your average score than ten strokes below, the curve will tend to have an extended right tail. Choosing your best 10 of your last 20 scores removes 'the right tail' from the calculation.
Best Ten versus your Average
Choosing your best 10 scores has several other implications. The first is that we are no longer talking about your 'average performance.' We are now talking about your potential - not your absolute potential, but your potential score on a 'good day.' The second implication is that because you are more likely to have a score towards the center or average of the bell curve, your next score is more likely to raise your handicap than to lower it (unless you 'improve' your average, which is one reason why it pays to get lessons - it moves the center of your bell curve to the left).
The Second Part
The second part of the formula (represented by the summation sign) averages these best ten differentials and multiplies the total by 0.96. Why 0.96? We're not quite sure. The best answer we have is to compensate better golfers for having narrower bell curves. Imagine two golfers: Player A, who has a handicap of 3, and Player B, who has a handicap of 15. Player A may shoot an average of 75, with a best score being 72 (a range of three strokes), while player B may shoot an average of 92, with a best score of 84 (a range of 8 strokes). Player B's range is more than 2x that of Player A's. This also means that Player B has the same potential of shooting twice as many strokes below his/her handicap as Player A. This translates into a distinct advantage in an odds sense.
The 0.96 scales back that advantage. If the average of the differentials for a player is 3.2, multiplying that number by 0.96 yields 3.0, a 0.2 stroke reduction. In contrast, if the average of the differentials for a player is 16, multiplying by 0.96 yields 15.3, over three times the reduction. In a game where every stroke counts, the extra strokes taken off of higher handicap players tries to even out the advantage.
Why 0.96 and not 0.95? It is an arbitrary choice. Five decades ago it used to be 0.86. We assume the USGA uses a number that seems to work best for them.
The final result is a handicap that is one of the best means of calculating odds between players. At the same time, because it is calculated against a benchmark of zero (scratch), it also serves as an accurate measure of your potential and level of game.