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Hmmm.
100 horses that went off at 2/7
100 horses that went off at 100/1
Which set of results from them would be the most reliable?
Obviously the first one, which means that if you want to "count" the number of runners to check if a sample size is valid you have to take into consideration the price.
I would recommend reading Steve Tilley's notes on sample size and stats 'usefuleness'.
For sample size you want to choose a consistent figure such as 10.0 and for usefulness/validity you want an Archie score of say 5+
Then there is sandboxing. You want to test data with past results, and then wait until a method has had sufficient live runs before continuing.
Sample Size
Sum up all the odds of the runners so for a 2/1 shot, a 3/1 shot and a 5/1 shot you would have:
1/(2+1) + 1/(3+1) + 1/(5+1) = 0.75
That figure (called the expected number of wins or Exp) is way too low. You want 10 be safe. Now you can see that if your method has mostly 100/1 shots then this is going to take a long time. But if your method contains short favourites you do not need "100's" of runners.
Archie Score
This is definitely something you want to calculate. It tells you how likley the results you have observed are due to luck or chance.
It is based on chi-squared and the lower the figure the more likely the results are BS, but the higher the figure the more likely you the figures are trustable.
Historic / Live
This is the hardest bit because anyone who has run a method and the initial figures are good, and the sample size (Exp) is great, and the Archie score is valid will totally convince themselves they are a 49er who started digging in California in 1847.
Unfortunately there are negative forces such as back-fitting and of course there is iron pyrite.
To protect against that you have to a) not back-fit at all (the hardest to avoid for the masses), b) sandbox your method.
To sandbox you have to research your idea and come up with a valid Exp and Archie figures. Then you sit on the results and observe what happens in the future. If your Live (future) Exp and Archie figures are valid then you should have a winning method and you should now start backing the method.
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