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.

Improving Beaten favourites -system

A while back I was reading through old Smartsig issues. One of the features there is simple to operate systems called KISS’s. In issue 0701 there was a mention of a system about beaten favourites, but it didn’t haven any data to back it up. Long story short, sounded interesting and wanted to give it a go.

Rules of the system are simple, back any beaten favourite which improved between second to last time out and last time out.

I took data from 2012 and 2013 and results are presented below.

Average odds22.40
Median odds8.96
Return on Turnover %2.52%
Std deviation4.10

A small profit over two years, but according to p-value, there is still over 12% chance that theses results are due to luck alone. And it can be considered that results are not statistically significant. Chart below show how bank shrinks and grows throughout the selections. Over two year the selections are mostly on making losses.