Scratches and ML Adjustments

[schorses]

Curious how everyone handles adjusting the morning line when there are scratches (thinking about use case for multi-race).

If a race has something like this for ML; 8/5, 3/1, 4/1, 6/1, 10/1, 15/1.

Case 1 - 8/5 scratches
Would you just equally distribute the odds to all the other horses, or maybe shift all the horses down, i.e 3/1 becomes 8/5, 4/1 becomes 3/1, etc?

Case 2 - 3/1, 4/1 or 5/1 scratch
Is this an easier case in that you would just redistribute the ML odds, or do you have to look at more than that?

Case 3 - 15/1 scratches - Seems like minimal impact and could just redistribute odds?


Case 4 - Race moves from Turf to Dirt - Do you just throw out ML in that case?

[jay68802]

IMO, the ML does not need adjusted in any way, under any of the circumstances you used. You should just be adjusting what the acceptable odds on your horse is. ML means next to nothing once the pool is open.

[CBYRacer]

I'll work through your first example and give you the rest for homework.

Step 1: Based on the original M/L (pre-scratches), determine what each horses' implied win probability is by using the following formula: Implied probability = 1 / (1 + odds). So, for the 3-1 for example, the implied probability is 1 / (1 + 3) = 0.25. Sorry if you already know this. Here are the implied probabilities based on your example

Odds Implied probability
1.6 0.38
3 0.25
4 0.20
6 0.14
10 0.09
15 0.06

Step 2: Take out the horse(s) who scratch. In this case, I took out your 6-5 shot

Odds Implied probability
3 0.25
4 0.20
6 0.14
10 0.09
15 0.06

Step 3: For each horse, take the horse's implied probability (e.g., 0.25 for the 3-1 shot) and divide it by the sum of implied probabilities for all horses. In this case, for the 3-1 shot you'd take 0.25 and divide by (0.25 + 0.20 + 0.14 + 0.09 + 0.06) and get 0.34. This represents the 'adjusted' probability after removing the scratch(es). Here are the 'adjusted' probabilities in your hypothetical example:

Odds Adjusted probability
3 0.34
4 0.27
6 0.19
10 0.12
15 0.08

Step 4: Finally, convert the adjusted probabilities back into odds by using the following formula:
Odds = (1-Probability) / Probability.
So, for the original 3-1, the adjusted odds would be (1-0.34) / 0.34 = 2-1
The full adjusted M/L therefore in your case 1 would be:

Original ML Adj. M/L
3.0 2.0
4.0 2.7
6.0 4.2
10.0 7.2
15.0 10.9

Case 2 would be the same methodology.

Case 3, same methodology as above and your intuition is correct. The impact will be lower. Think of it this way...if a longshot scratches, any given horse's chances of winning go up (and hence odds go down), but not by that much

Case 4: I wouldn't throw away completely. There's still going to be some information in the M/L even if the race switches surfaces. Though, not as much as before