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Originally Posted by raybo
Ok you stats gurus, if I have a rating derived from multiple factors, each weighted according to significance, how do I get from that final rating to a projected win probability/percentage?
Let's start with the following final ratings for a race:
#1 -- 72.65
#2 -- 101.06
#3 -- 104.32
#4 -- 87.72
#5 -- 89.90
#6 -- 10.20
#7 -- 75.89
#8 -- 79.96
#9 - 105.25
#10 - 77.95
#11 - 73.84
#12 - 69.82
How do I get from those ratings to a calculated/projected win probability/percentage? I have always thought that you just divide each rating by the sum of all the ratings, but can't seem to find anything related to this type of calculation on the web, and I haven't tried to create a line in a long time, so I'm a bit rusty.
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Here is a simple method I posted many years ago that I still use often. It helps to have a cutoff, i.e. a spread that anything below is not considered a contender for the race. That is something only you would know what is best and pretty easy to test if you store your data as I imagine you do. For an example, I'll use 16 for your set. (Of course all this can be done with a program or spreadsheet, but I'm showing it manually here to show the details)
The top rating is 105.25, so to be included as a contender you need to be within 16 or 89.25.
That leaves four horses:
#2 -- 101.06
#3 -- 104.32
#5 -- 89.90
#9 -- 105.25
The minimum points I give a contender are 2. That number could vary for you and others just like the cutoff number. So I take the Min Contender rating, 89.90, deduct 2 = 87.90, and subtract that number from each rating.
#2 -- 13.16
#3 -- 16.42
#5 -- 2.00
#9 -- 17.35
I then add these up for a total of 48.93. Next, divide each rating into the total for a percentage:
#2 -- .27
#3 -- .34
#5 -- .04
#9 -- .35
Next, I account for the non-contenders. Some people like to assign a blanket number like 20%. I do it a little different but that is individual preference. I take the number of contenders, four in this case, and divide it by the field size, 33.333% in this case. I then average it with 100%. I find if I pick only four contenders in a 12 horse field I'm crazy if I think I'll have the winner 80% of the time. So in this case I use 66.6666% as my contender percentage. I then adjust the win percentages above by this estimate
#2 -- .27 / .66666 = .18
#3 -- .34 / .66666 = .23
#5 -- .04 / .66666 = .03
#9 -- .35 / .66666 = .23
I then convert to an odds line:
#2 -- .18 = 4.55
#3 -- .23 = 3.34
#5 -- .03 = 32.33
#9 -- .23 = 3.34
That is my fair odds line. I rounded here since I did it manually, will come out a little different if you program this, but that is the gist of it.
Oh, last thing, any horse with a "fair odds line" > than natural odds is tossed by me from the contender list, but again that is just from my experiences and numbers. Your mileage may vary.
