Oppenheim decided to divide the population of prizemoney earners in a particular season in each jurisdiction, or country, into three bands. The top 2% of earners are the "A runners" , the next 2% are the "B runners" and the next 4% are the "C runners" . In other words the A+B+C runners combined make up the top 8% (2+2+4) of earners in a particular jurisdiction, country, each season.

This masterstroke achieves a number of objectives in one. It largely removes the bias in prizemoney based lists created by a relatively small number of very rich races. It largely removes the regional and political element of stakes races which could be either a regional bias or a national one. Finally it helps to compare stallion merit across jurisdictions, or countries, with vastly differing prizemoney levels.

The ratings are calculated in the following way. "A" runners are the top 2% of prizemoney earners in a season, so if a particular stallion has 2% of his runners in a particular period among the top 2% of earners his APEX A rating figure for that period is 1. If he has 10% of his runners among the top 2% his APEX A rating figure is 5. "AB" runners are the top 4% of earners so if a stallion has 4% of his runners among the top 4% of earners his "APEX AB rating figure is 1. "ABC" runners are the top 8% so if a stallion has 8% of his runners among the top 8% of earners his "APEX ABC rating is 1. If he had 16% of his runners among the top 8% his APEX ABC rating would be 2. And so on. The ratings can be done in any band individually or in any combination.

To give you an idea as to what standard of horses are included in the ratings, in the second last completed season in Australia (2005/06), in that season the top 2% of runners earned over $93,300, the next 2% of runners earned between $55,450 and $93,300 and the next 4% of runners earned between $30,640 and $55,450. In the 2005/06 Australian racing Season there were 31,193 runners, which means there were 619 "A" Runners (2%), 1237 "A+B" runners and 2475 "A+B+C" runners.

In addition to the earnings, to qualify for a rating on the main list a stallion must have a predetermined minimum number of starters across a seven year band. The most commonly published lists are based on a minimum of what are called "150 year starters". If a runner by a particular stallion earns prizemoney in each of the seven years under review he will count as 7 of the 150 "year starters".

Lists can be compiled for a smaller number of starters, also for first and second year sires etc. but will of necessity be less accurate than the main list. So, with APEX ratings, as with any type of sire lists, some skill in interpretation is still required.

Perhaps the main thing to be aware of is that some young sires and some sires with limited numbers of runners will not necessarily appear on the lists at all. This does not mean they are not good sires, because when they do make the list, i.e. qualify with sufficient "year starters", they might very well enter at, or near, the top.