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%