Let's have a look .......... 2.5 million runs, looking for RF's <1 (in a VERY general sort of way)

RFS........ <3 or >11
Career .....>19 races
Age .........>5
Wt Carr ...<55 kg
DSLS .......>25 d
WSR........ <15%
PSR..........<35%
Sum L4 Places .... >19
Sum L4 BD's ..... >16

Now you might find a horse with all these characteristics once in a lifetime. Alternately you might find a horse like Jester's Girl (1/12/2012 Eagle Farm R5#4) that couldn't win even if it grew a fifth leg ... it didn't care and won anyway at $60, and there are a squillion other examples of "bad luck".

I have played with and researched the maths of "laying" methods for many years and have, sadly, come to the conclusion that if you're not a bookmaker you're going to get shafted by the Jester's Girls of this world.

Cherry picking lays is a dream :-)