Impact value is the pecentage of winners with the factor -- the percentage of ALL winners in a sample with the factor divided by the percentage of all the starters with the factor.
So, let's say you have a sample of 1000 races and you find 50 plays. Let's say you have 13 winners out of the 50. And let's say the total number of starters in the 1000 races is 8000 horses.
IV = ( %winners with factor ) / ( % starters with factor )
= (13/1000) / (50/8000)
= 0.013 / 0.00625
= 2.08
Odds don't come into it.
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