Hi Possie,
You are asking for just one single thing that improves possible profit.

I can answer the single question with a single response that can work.
It's the answer you may not like.

Your question is...
What individual factor will improve a selection method?

Answer ....
Delete any selection that is not in the top 2 in the Betfair market at jump time.

The top 2 favs produce 50% winners.

I wont answer as to what races this best applies to because you only want a single factor .

It usually takes more than one factor to improve returns but this is a start .

Cheers.

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Cheers.