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.

Accuracy of Betfair starting prices

I have a feeling that I am going to have a need to refer to accuracy of Betfair starting prices in my future articles. For that reason I decided to create nice chart and table that compares the expected strike rate and actual strike rate in different odd ranges.

In odd ranges I used the fractional odd increments as limits for a range. Data covered is all races ran in UK and Ireland in 2013. And I decided to limit analysis this time to odds under 10.0.

Let’s start with pretty pictures and present the data in line chart form where we can see that actual strike rate follows the expected strike rate pretty well, biggest differences are in the shorter odds, especially those under evens but after that it balances out and differences are not too big.

And then the same in table form.

FractionalDecimalExp.SRAct.SR
1/41.2585.26%81.73%
1/31.3377.38%70.59%
2/51.473.03%71.72%
1/21.569.03%64.14%
4/61.6763.05%61.61%
1/1254.27%55.92%
11/102.148.98%48.26%
6/42.543.29%42.04%
7/42.7538.1%38.61%
2/1334.77%35.3%
9/43.2532.08%31.42%
5/23.529.68%28.32%
11/43.7527.62%27.38%
3/1425.85%25.84%
10/34.3324.02%22.79%
7/24.522.67%23.52%
4/1521.12%21.96%
9/25.519.11%19.34%
5/1617.44%17.34%
11/26.516.05%16.16%
6/1714.85%14.85%
13/27.513.84%13.66%
7/1812.94%11.21%
15/28.512.16%12.11%
8/1911.45%12.24%
17/29.510.84%10.48%
9/11010.27%10.28%