9 hole CH calculation

sawtooth

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The 9 hole calculation in HM seems to be

Course Handicap = (Handicap Index ÷ 2) x (9-hole Slope Rating ÷ 113) + (9-hole Course Rating – 9-hole par)

Am I right in thinking that the 9 hole calc had already included Cr-Par for some time?

This isn't the case yet for 18 holes.

Players are asking why their 9 hole handicaps seem mean, less than half their 18 hole ones, even though CR and slope is comparable (almost exactly half Course Rating).

I think the Cr-Par is the bit to explain the difference.
 

Jimbof

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The reason 9 hole hcps included CR-Par was due to the approach for scaling up - i.e. Net Par + 1 (or 17 points). That would only be valid if the play-to-handicap score on the 9 holes played was 18 points. If it wasn't 18 points, the scale-up score would be variable, hence CR-Par is used to address that. In addition, even under CONGU CR-Par was used for 9-hole scores for the same reason.
 

wjemather

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That's what I thought. But how is it that Cr-Par is already in the 9 hole formula but not included for 18 holes?
It was entirely for "constancy" with how 9-hole handicaps were (or were supposed to have been) calculated before WHS.
CONGU mistakenly thought it would be overly confusing if they changed too much too soon, so they selected an option that was time-limited - the option to not use CR-Par expires in 2028, so would have been mandatory with the next update.
 

D-S

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It was entirely for "constancy" with how 9-hole handicaps were (or were supposed to have been) calculated before WHS.
CONGU mistakenly thought it would be overly confusing if they changed too much too soon, so they selected an option that was time-limited - the option to not use CR-Par expires in 2028, so would have been mandatory with the next update.
I heard that comment at the seminar too, I wonder if EG knew this on WHS introduction in 2020 and were always going for a two stage implementation or that they didn’t at the time and were going to stick with their original intention forever.
Either way they admit it was a misstep and it’s rectified now.
 

wjemather

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I heard that comment at the seminar too, I wonder if EG knew this on WHS introduction in 2020 and were always going for a two stage implementation or that they didn’t at the time and were going to stick with their original intention forever.
Either way they admit it was a misstep and it’s rectified now.
I got the impression from those in charge at the time that CONGU knew there would be moves towards standardisation on many options, even if they had no timetable.
 
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LizAig

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That's what I thought. But how is it that Cr-Par is already in the 9 hole formula but not included for 18 holes?
At the WHS forums they said EG decided not to introduce it for 18 holes because they didn’t want to confuse people….which seems daft given people were already confused by a completely new system so what difference would that bit have made!! They said they always planned to introduce it…..i really think we could have coped with it being there from the start!!
 

bodand

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Followup question please on the new 2024 WHS rule for generating an 18 hole DIFF after playing only 9 holes. Easiest to ask by way of an example:

1) I have an index = 14

2) I play a 9-hole round and shoot 44 (adjusted) on a course with the 9-hole data: CR=35.5, CS=138, PCC=0.

3) The 9-hole DIFF from that round is 6.96 = (44-35.5) / (138/113) = 8.5 / 1.2212.

4) This site below reports the same 6.96 (7.0 rounded), and also reports an 18-hole DIFF = 15.4.
http://golf.okrasa.eu/language/en/handicap-en/whs-en/score-differential/#Calc
If I were to just assume the missing back 9 is replaced by “replicating” the same front 9, I would have a 18 hole
DIFF = 13.96 = (88-71.0) / (138/113) = 17 / 1.2212.

5) So Question 1 is: why the heck did I get punished with the 15.4 DIFF? Just assume “duplication” I’d have a 13.96.

6) Question 2: how does one calculate that 18 hole DIFF? I can’t find the math formula, only “add the front 9 DIFF to the player's EXPECTED 9-hole DIFF" to get the final 18 hole DIFF.

5) The USGA site below gives similar example:
A player with a Handicap Index of 14.0 posts a 9-hole score of 41. Based on Course
Rating™ and Slope Rating™ of the tees played, this results in a 9-hole Score
Differential of 7.2. That value is then added to the player's expected 9-hole
Score Differential to determine an 18-hole Score Differential of 15.7

 
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Alan Clifford

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Followup question please on the new 2024 WHS rule for generating an 18 hole DIFF after playing only 9 holes. Easiest to ask by way of an example:

1) I have an index = 14

2) I play a 9-hole round and shoot 44 (adjusted) on a course with the 9-hole data: CR=35.5, CS=138, PCC=0.

3) The 9-hole DIFF from that round is 6.96 = (44-35.5) / (138/113) = 8.5 / 1.2212.

4) This site below reports the same 6.96 (7.0 rounded), and also reports an 18-hole DIFF = 15.4.
http://golf.okrasa.eu/language/en/handicap-en/whs-en/score-differential/#Calc
If I were to just assume the missing back 9 is replaced by “replicating” the same front 9, I would have a 18 hole
DIFF = 13.96 = (88-71.0) / (138/113) = 17 / 1.2212.

5) So Question 1 is: why the heck did I get punished with the 15.4 DIFF? Just assume “duplication” I’d have a 13.96.

6) Question 2: how does one calculate that 18 hole DIFF? I can’t find the math formula, only “add the front 9 DIFF to the player's EXPECTED 9-hole DIFF" to get the final 18 hole DIFF.

5) The USGA site below gives similar example:
A player with a Handicap Index of 14.0 posts a 9-hole score of 41. Based on Course
Rating™ and Slope Rating™ of the tees played, this results in a 9-hole Score
Differential of 7.2. That value is then added to the player's expected 9-hole
Score Differential to determine an 18-hole Score Differential of 15.7



6.96 +14 * 0.52 + 1.2 = 15.44.
 

rulefan

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Followup question please on the new 2024 WHS rule for generating an 18 hole DIFF after playing only 9 holes. Easiest to ask by way of an example:

1) I have an index = 14

2) I play a 9-hole round and shoot 44 (adjusted) on a course with the 9-hole data: CR=35.5, CS=138, PCC=0.

3) The 9-hole DIFF from that round is 6.96 = (44-35.5) / (138/113) = 8.5 / 1.2212.

4) This site below reports the same 6.96 (7.0 rounded), and also reports an 18-hole DIFF = 15.4.
http://golf.okrasa.eu/language/en/handicap-en/whs-en/score-differential/#Calc
If I were to just assume the missing back 9 is replaced by “replicating” the same front 9, I would have a 18 hole
DIFF = 13.96 = (88-71.0) / (138/113) = 17 / 1.2212.

5) So Question 1 is: why the heck did I get punished with the 15.4 DIFF? Just assume “duplication” I’d have a 13.96.

6) Question 2: how does one calculate that 18 hole DIFF? I can’t find the math formula, only “add the front 9 DIFF to the player's EXPECTED 9-hole DIFF" to get the final 18 hole DIFF.

5) The USGA site below gives similar example:
A player with a Handicap Index of 14.0 posts a 9-hole score of 41. Based on Course
Rating™ and Slope Rating™ of the tees played, this results in a 9-hole Score
Differential of 7.2. That value is then added to the player's expected 9-hole
Score Differential to determine an 18-hole Score Differential of 15.7

The first link almost certainly will not have the WHS (ie USGA) formula for calculating the Expected Score / 'expected differential' for the unplayed 9

The WHS (USGA) formula for calculating the Expected Score / 'expected differential' is not published and is not available outside the USGA/R&A.

What is an “Expected Score”?
An expected score is the score a player is expected to achieve over a specified number of holes on a course of standard difficulty. It is based on the average Score Differential of players with a given Handicap Index and a normal distribution of scores. By using expected score to convert a 9-hole score into an 18-hole Score Differential, good 9-hole rounds (relative to the player’s ability) will result in good 18-hole Score Differentials, average 9-hole rounds will result in average 18-hole Score Differentials, and so on.
 

Voyager EMH

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The first link almost certainly will not have the WHS (ie USGA) formula for calculating the Expected Score / 'expected differential' for the unplayed 9

The WHS (USGA) formula for calculating the Expected Score / 'expected differential' is not published and is not available outside the USGA/R&A.

What is an “Expected Score”?
An expected score is the score a player is expected to achieve over a specified number of holes on a course of standard difficulty. It is based on the average Score Differential of players with a given Handicap Index and a normal distribution of scores. By using expected score to convert a 9-hole score into an 18-hole Score Differential, good 9-hole rounds (relative to the player’s ability) will result in good 18-hole Score Differentials, average 9-hole rounds will result in average 18-hole Score Differentials, and so on.
Like I said.
 

bodand

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Hi Y'all - Thanks for the chat above.
I made this table of "expected scores" just by using that cool OKRASA link above (http://golf.okrasa.eu/language/en/handicap-en/whs-en/score-differential/#Calc).
Odd that it is not published, since it is easily extracted like below. I guess my initial gut reaction that my 14 index got "punished" with an expected score DIFF of 17 (for 18h) or 8.5 (for 9h) was stupid of me... I was forgetting that the index is the best 8 of my last 20, and the the "expected" score is the average of all scores of players of my index (not my best ones). Not sure I love the new rule for making 9 holes scores immediately into 18 hole scores (think i liked the old system better), but at least I understand it now.
Very much appreciated.
PLAYER INDEX
0​
1​
2​
3​
4​
5​
6​
7​
8​
9​
10​
11​
12​
13​
14​
15​
16​
17​
18​
EXPECTED 9-HOLE DIFF
1.2​
1.7​
2.2​
2.8​
3.3​
3.8​
4.3​
4.8​
5.4​
5.9​
6.4​
6.9​
7.4​
8​
8.5​
9​
9.5​
10​
10.6​
EXPECTED 18-HOLE DIFF
2.4​
3.4​
4.4​
5.6​
6.6​
7.6​
8.6​
9.6​
10.8​
11.8​
12.8​
13.8​
14.8​
16​
17​
18​
19​
20​
21.2​
(EXPECTED 18-HOLE DIFF) - INDEX
2.4​
2.4​
2.4​
2.6​
2.6​
2.6​
2.6​
2.6​
2.8​
2.8​
2.8​
2.8​
2.8​
3​
3​
3​
3​
3​
3.2​
 

bodand

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My understanding that there is no "mathematical formula" for the "expected DIFF" for a given index, just a collection of tabular data that USGA/R&A use. I just reverse engineered that table from the OKRASA web site. I do not know the veracity of that site, but it seems pretty trustworthy. As expected that web site shows "expected DIFF" independent of course slope, course rating or front-9 score..... it is solely a function of player index. The author's web page on that site says he is a member of handicapping and course rating committee of the Polish Golf Union. I sent him the link to this chat...

Good point Alan C, "expected 18-hole DIFF' was just my construct (= 2 x "9-holes expected DIFF") in table above to help me understand what "expected DIFF" means (by relating to INDEX, ie, showing it is "INDEX plus a couple").... Apologies!

My main conclusion is: want to know a "9-hole expected DIFF" used to finish your unplayed back 9? Just add 1.5 to your INDEX/2 and you will be very close.
 
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DaOk

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I sent him the link to this chat...

I was called to answer, so here I am.
The 18-hole Score Differential of 9-hole round used for handicap purposes is the sum of the 9-hole Score Differential and the Expected Score (for the second nine).

The 9-hole Score Differential = (113 / 9-hole_SR) x (9-hole_Adjusted_Gross_Score - 9-hole_CR) - Rules of Handicapping, Rule 5.1b

The formula for the Expected Score in general is rather complicated and probably therefore it is not published. But for 9-hole round it is simple:
9-hole Expected Score = (0.52 x Handicap_Index) + 1.2

Both values are not rounded before summing. Finally the calculated 18-hole Differential Score is rounded to one decimal place.

That's all. I hope I helped.
 
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