The selections are done automatically by the computer and do not adjust for things like:
and uses up to the last 20 ratings without adjustment (and so new performers moving through the grades often rate lower). Only raced horses are rated so forget about early 2yo's and other races consisting of mainly first or lightly raced runners.
After scratchings, over 80% of all winners are found in the seleceted top five selections using the Pureform database on races where the number of first starters or lightly raced horses is less than 20%. For these selections, the top six picks are included before scratchings...
To improve your chances, some fine tuning is needed:
In most cases, selecting the best of the six horses is not a difficult task.
Good luck with the auto rate selections.