A/E
The A/E index indicates if a statistic is good or bad and is calculated as follows:-
A = Actual Winners
and
E = Expected Winners.
A is easy to calculate as it is the number of winners.
E is the total of all the probability or odds of winning for each horse based on the Betfair SP (minus any commission) prices for each horses
e.g. If we have 100 horses who went off at 3/1 then we would expect 25 of them to win. The odds of a 3/1 shot is 0.25 so adding all 100 of those together = 25.
Once you have the expected number of winners then you just divide actual winners by expected winners to produce the A/E index.
An A/E index of 1.00 means the statistic is performing as expected, less than 1 and it is under performing and more than 1 and it is performing better than expected.
As a benchmark an A/E of 1.25 or greater would be very good and an A/E of 0.75 or lower would be bad.