JJobu

I checked a few others and all seem to fall into the 1.4 to 1.45 range added to half my index at the time of posting. With the first one from my last post I think the reason for the difference is I had a score the day before that did not trigger an index calculation the next day. Either I forgot to post the day of or the system was delayed in adding the revision, both of which have happened before. So the system may have gone back and adjusted based on the delayed posting/revision and my index would have been 10.4 not 10.7 which would make that one 1.371 added to half my index.

Looking at it your way of explaining it is my math off here? It doesn't seem to be the same each time. I understand there would be minor differences as my index fluctuates up or down a moderate amount but with only a range of 10.7 to 10.9... They are probably rounding at some point or maybe multiple points. 4/16 index = 10.7 (half is 5.35) score of 42 on a 34.0/114 course = 14.5 differential (42 - 34.0) * (113/114) = 7.929 7.929 + 5.35 = 13.279 14.5 - 13.279 = 1.22 added to half my index 5/8 index = 10.8 (half is 5.4) score of 39 on a 33.7/121 course = 11.8 differential (39 - 33.7) * (113/121) = 4.949 4.949 + 5.4 = 10.349 11.8 - 10.349 = 1.45 added to half my index 5/16 index = 10.9 (half is 5.45) score of 40 on a 34.0/114 course = 12.8 differential (40 - 34.0) * (113/114) = 5.947 5.947 + 5.45 = 11.397 12.8 - 11.397 = 1.40 added to half my index

We really can't get at the formula unless we had a large number of scores and knew the exact index of the player when they posted the score. It is taking the actual 9-hole differential and adding another differential for the other 9-holes which is the average differential (not score) for someone of the same index at the time of posting. 3 players with indexes of 10.1, 10.2 and 10.3 shooting the same score, from the same tees, might produce 3 different differentials. For my course the differential produced for 9-holes played is like I shot 41 on the 9-holes not played. Doesn't matter if I played the front 9 or back 9. Adding 41 to any of my 9 holes scores and comparing that 18-hole score differential to the 9-hole differential when posting gets me to within +/- .3 for almost all scores with one exception where the variance is +/- .5. Of course as my index changes more so should the expected differential for the 9-holes not played.

My understanding is that it uses an expected score based on an average score, for someone of your index, for the 9-holes not played and using the slope and rating for the 9-holes not played. edit: in short it adds the expected remaining 9-hole score to the score of the 9-holes you did play and then using that 18-hole score to calculate the differential. My last 2 scores are : 80 for an 18-hole round with a differential of 11.0 39 for a 9-hole round with a differential of 11.0 (which means the Expected Score for someone at my index for the back 9 at my course is a 41). ie. a 18-hole round of 80 is the same 11.0 differential.

Edit: I do not think what I typed below is quite correct. See my post lower down in the thread. I assume using the Expected Score and then the normal formula to calculate the remaining 9-holes not played. For a slope of 125 the formula would be... ie. Handicap Differential = (Expected Score - back 9 Course Rating) x (113 / 125) = X

We have now had a month of posting 9-hole scores under the new system which is a decent sample size for my course as maybe 50% - 60% of the rounds posted by players in my group (25'ish players) are 9-hole scores rather than 18 and almost all of us play multiple times per week, every week. The general consensus seems to be that indexes feel inflated. That makes sense considering the Expected Differential for the 9-holes not played is based on an average for players in the same handicap range. It eliminates the possibility of a 9-hole differential ever being posted as 2x9-holes of better than average golf which I am not convinced is the best way to approach it. I am working under the assumption that the Expected Differential is an overall average for people in the same handicap range rather than an average of only the scores that counted for handicap purposes. It would be interesting to see if there would be any difference between an average 9-hole differential for the back 9 of 18 hole rounds when the score counted for the handicap calculation versus just the Expected Differential which is an average of all rounds(?). At any rate, it seems like they are trying to make 9-hole rounds more comparable to 18-hole rounds but I am not sure they are approaching it the best way but of course I have no hard data to back that up. Just off the top of my head I get the feeling that forcing players to post a minimum of the front 9 score and back 9 score for 18-hole rounds (versus allowing just the overall score) and then using each 9-hole score as a 9-hole differential would put things maybe more in line. ie. your index would be the best 16 of 40 x 9-hole differentials x 2. The overall 18-hole index then might need to be multiplied by a small multiplier, like 1.02 for an 'out of thin air' example, so that indexes remain more in line with how they have been in the past.