How accurate are overnight Betfair prices?

For one system test I am running I have been collecting early Betfair prices the night before for all of the selections before making the final decision. And I started wondering that how accurate they are. I only have data for little over 800 selections and they are part of currently profitable set of selections so that might color the results but still, I am surprised at the accuracy of the early odds. Chart over different odds ranges is shown below.

Actual number winners over the data is 179 and expected number based on early odds is 183 while expected number of winners based on BSP is only 162. Well, that shouldn’t surprise as system wouldn’t be making profit against BSP unless it was  beating the accuracy of BSP.

I need to see if I can get hold of more and more complete set of data that wouldn’t be coloured by my selection process to be able to do more throughout analysis.

This subject actually touches on very first article I wrote for SmartSigger about accuracy of Racing Post forecast and if it would be possible to make profits when betting against the public.