Well they can be as simple or as complex as you like - for example when attending the races I often do a simple rating of each runner by averaging a) 2nd last start figure minus last start figure & b) this start weight minus last start weight - the highest figure is a selection if within the first 5 or so of betting & has a good jockey up.

Now of course this loses longterm but I've had plenty of fun with it over the years, add a couple of other filters & it does alright.

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"Now let me get this straight - Whatever I do don't bet this horse?"