In this penultimate Derby post I will look at a method that not only allows me to combine the various features studied so far but also take account the distinct possibility that a big priced winner may be distorting those profit and loss figures for an individual element. At the conclusion of the next blog entry we will have a rudimentary set of ratings for the Derby that will allow us to rank and hence make some selections based on the ratings.
An AE value is an actual divided by expected value. Let me illustrate how they work by examining the high,low and mid draw features. If we take all horses over the last 10 years that were drawn low and count up how many won the Derby we have a number which we will call actualWins. Now if we take the Betfair SP of each one of the low draw runners and calculate the chance of winning according to each Betfair SP (for example a BFSP of 2.0 would be a0.5 chance, a BFSP of 3.0 would be a 0.33 chance) and then add these up we have what I will call expectedWins. If we now divide actualWins by expectedWins we have our AE value for the low draw. Any AE value above 1 means more horses won than the odds suggested and any value below 1 means fewer horses won than the odds suggested. The AE value can paint a more accurate picture because it takes into account the odds unlike flat level profit and loss reporting. It also gives us an indication of whether the public is over betting or under betting a feature. In the long run this is more important to us than finding features that predict winning or losing (yes I said it).
Here is the AE calculation for a low draw
actualWins = 1
expectedWins = 3.07
AE value = 0.325
Clearly low draw horses have been winning at a far lesser rate than their odds would suggest although the small sample size we are using here means we should approach this number with some caution. It is likely however given our knowledge of the conformation of the course that the AE value is somewhere below 1
The AE values for mid and high draws are as follows
Mid draw AE = 1.5
High Draw AE = 1.02
Notice how the AE values somewhat contradict the profit and loss figures in the previous blog where high numbers produced by far the best profit, no doubt swelled by a big priced winner.
In the final blog entry before the Derby I will put all the pieces together to produce a set of ratings.
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