I am trying to build a system. I have results starting from 15 March this year to now. 533 selections.
return 79% for win and 83% place using Unitab prices.

Analyzing the results seems to throw up some odd things.
eg.
Race No.3 44 selections 16% S/R for a return of 39% at Unitab prices
Race No.2 42 selections 45% S/R for a return of 131% unitab prices.

Looking at these two races closer
R2. 50% of the winners are three year olds (they make up 28% of all the selections in R2)
R3. only 13% winners are three year olds but they make up 50% of all the selections

So the Question is do you (going by statistics) back all three year olds in race two and lay them in race three? or do you need more results?