According to the USGA website a Score Differential ‘measures the performance of a round in relation to the relative difficulty of the course that was played, measured by the Course Rating and the Slope Rating. The result of the daily conditions calculation (PCC) is also included in the Score Differential calculation which may provide an adjustment if course and/or weather conditions significantly impacted scores on that day’.
A Score Differential is calculated using the following formula:
(113/Slope Rating) x (Adjusted Gross Score – Course Rating – PCC adjustment)
There’s quite a lot to unpack here, but the first thing to understand is that your score and your score differential are different. The good thing is that the Score Differential is calculated for us and it is there on our app, eg England Golf, and we can see our last 20 scores and what the Score Differential is. It’s when we look at all our scores that hopefully things become a bit clearer and our best eight scores are highlighted so we can see when a good/bad score will drop off.
To understand what each part is, put simply, Slope Rating ranges from 55-155 with the average being 113. When you play a course with a Slope Rating higher than 113 your Course Handicap will be higher than your Handicap Index and when you play a course with a Slope Rating lower than 113, your Course Handicap will be lower than your Handicap Index.
The Slope Rating will vary from whichever tees you play from. It’s worth pointing out that the higher the slope rating doesn’t necessarily mean that the course is more difficult rather that the greater the difference is expected between the scores of a scratch and bogey golfer (see below).
The Adjusted Gross Score is a player’s score that is adjusted for when a player has a big number or doesn’t play a hole and a Net Double Bogey is awarded so you can have nothing on your card higher than that score.
The Course Rating is the evaluation of the playing difficulty of a course for the scratch handicap (0). It is used in conjunction with the Bogey Rating (the difficulty of the course for a bogey player of 20-24 handicap) to determine the Slope Rating. The Course Rating system is extremely objective and considers more than 460 variables on a standard course rating, from each set of tees.
The PCC (Playing Conditions Calculation) determines whether playing conditions on the day differed from normal conditions to the extent that an adjustment is needed to compensate.
Going back to the original question a Score Differential must be an 18-hole value or its calculated equivalent.
So when we take our eight best Score Differentials (not scores) from our last 20 rounds and find an average that determines our WHS Index. Like most things, particularly when it comes to disabilities, it really is a case of trusting the system and involving yourself in it and then things become a lot clearer.