I am hoping someone with some mathematical probability knowledge will be able to help me here. I am looking at a possible favourites lay system but I need to know if the testing results I have are at least a semi-accurate indication of of the future. The system I am looking at considered 1000 races over the past 4 months and selected 70 races to lay the favourite, if I were to randomly select 70 races in a group of 1000 what would be a probable win percentage of favourites in those 70 races?

Cheers,

Matt.