AEI And Comparable Index: Two Numbers Quarter Horse Breeders Should Understand

When Quarter Horse breeders evaluate a stallion, the first numbers they usually notice are total progeny earnings, stakes winners, winners and perhaps average earnings per starter.

There are two additional statistics, however, that can provide breeders with another perspective on a stallion's performance: Average Earnings Index (AEI) and Comparable Index (CI).

Both are available through statistical information provided by Equineline.com, and when viewed together, they can help answer an important breeding question:

Is a stallion getting better-than-average results from his opportunities?

What Is Average Earnings Index?

Average Earnings Index measures the earning performance of a stallion's runners relative to the average earnings of runners competing during the same period.

The key number to remember is 1.00.

An AEI of 1.00 represents average.

According to Equineline, the calculation is made for each calendar racing year in which a stallion has runners. Those annual figures are then averaged to produce his lifetime AEI.

That makes the number relatively easy to interpret.

A stallion with an AEI of 1.00 is essentially producing average earnings based on the population against which he is being measured.

An AEI of 1.50 means his progeny have performed at approximately 50 percent above the average earnings level.

An AEI of 2.00 indicates an earnings performance approximately twice the average.

Likewise, a stallion with an AEI below 1.00 has progeny earnings below the average represented by the index.

That distinction is important because AEI isn't simply another way of listing total progeny earnings.

A stallion with hundreds of starters can accumulate substantial earnings through sheer numbers. AEI attempts to put those earnings into context by asking how effectively his runners earned compared with the applicable racing population.

Why AEI Can Be Especially Interesting In Quarter Horses

For Quarter Horse breeders, the statistic can add another dimension to the familiar sire statistics of total earnings, stakes winners and average earnings per starter.

Consider two hypothetical stallions.

Stallion A has progeny earnings of $5 million and Stallion B has progeny earnings of $3 million.

Looking only at total earnings, Stallion A appears to be the more successful sire.

But suppose Stallion A has an AEI of 1.10 while Stallion B has an AEI of 1.75.

That changes the discussion.

Stallion A's progeny may have earned more money collectively, but Stallion B's runners have produced earnings substantially farther above the average represented by the index.

Neither statistic makes the other irrelevant. Instead, they answer different questions.

Total earnings tell breeders how much the progeny earned.

AEI helps tell breeders how those earnings compare with the average.

Then Comes Comparable Index

This is where the evaluation becomes even more interesting.

A stallion does not produce his runners by himself.

The quality of mares bred to him can have an enormous influence on his eventual record at stud. A stallion receiving large books of accomplished mares and daughters of elite producers begins his career with advantages that another stallion covering a much less accomplished mare population may not enjoy.

Comparable Index provides context for the quality of the mares bred to the stallion.

That means breeders can look at the stallion's AEI alongside the Comparable Index associated with his mares and get another indication of how the sire performed relative to the opportunity he received.

The comparison can be revealing.

Suppose a stallion has:

AEI: 1.80 | CI: 1.30

His progeny earnings index is substantially higher than the comparable level represented by the mares bred to him.

From a breeder's perspective, that is an interesting relationship because the stallion's runners are performing above the benchmark suggested by his mare population.

Now consider another stallion:

AEI: 1.40 | CI: 1.90

An AEI of 1.40 by itself sounds good—the stallion is above the 1.00 average.

But the second number provides important context. His Comparable Index is considerably higher at 1.90.

In other words, breeders should at least investigate whether the stallion's respectable AEI is partly associated with the superior quality of mares he has received.

How AEI And CI Work Together

This is why simply searching for stallions with an AEI above 1.00 doesn't tell the complete story.

One potentially useful comparison is the relationship between AEI and CI. If a stallion consistently maintains an AEI above his Comparable Index, it provides evidence worth considering that he is getting strong results relative to his opportunities.

Consider three hypothetical stallions.

STALLION A — AEI 2.10 | CI 1.45

With an AEI 0.65 higher than his Comparable Index, Stallion A's progeny are performing substantially above the level suggested by the mares bred to him. That could be a particularly positive indicator for breeders evaluating his performance at stud.

STALLION B — AEI 1.70 | CI 1.65

The two indexes are nearly identical. Stallion B's progeny are performing approximately in line with the level represented by his mares. His AEI is still well above the 1.00 average, but the CI provides important context for that success.

STALLION C — AEI 1.40 | CI 1.90

Stallion C also has an above-average AEI, but his Comparable Index is 0.50 higher. That suggests his strong earnings record should be viewed in the context of the particularly strong group of mares he has received.

The important point is that all three stallions have an AEI above the 1.00 average—but the relationship between AEI and CI tells three very different stories.

That doesn't make Stallion C unsuccessful. Nor does the AEI-CI relationship prove that Stallion A is genetically superior.

It simply gives the breeder another piece of information.

A Quarter Horse Example Exists In Equineline's Own Reports

Importantly for Quarter Horse breeders, these statistics aren't limited to Thoroughbreds.

An Equineline sample Quarter Horse statistical report for A Revenant demonstrates how the measurements appear in an actual Quarter Horse sire report.

The report lists earnings, earnings per starter, earnings per start and Average Earnings Index, along with a Comparable Earnings Index. It also converts average racing distances into both furlongs and yards—an important distinction when analyzing Quarter Horse racing.

That demonstrates how AEI and CI can be incorporated into the statistical evaluation of a Quarter Horse sire rather than being regarded solely as Thoroughbred breeding statistics.

Don't Use AEI In Isolation

Like virtually every sire statistic, AEI has limitations.

One exceptional runner can have a significant impact on the earnings of a young stallion with relatively few starters. Quarter Horse racing can magnify that issue because individual futurities can carry enormous purses.

A young sire whose first crop contains the winner of a multi-million-dollar futurity could see his earnings statistics change dramatically.

That doesn't invalidate AEI. It means breeders need to consider the size and maturity of the sample.

The number of starters matters.

The number of crops to race matters.

The distribution of earnings throughout those runners matters.

So do stakes winners, stakes performers, winners from starters, earnings per starter and median earnings.

For Quarter Horse breeders, current-year and 2-year-old AEI figures can also be particularly volatile, simply because so much of the breed's purse structure is concentrated in major juvenile futurities.

Lifetime AEI becomes increasingly meaningful as additional crops and runners contribute to the stallion's record.

Another Tool In The Breeder's Toolbox

No single statistic can identify the right stallion for every mare.

Pedigree compatibility, conformation, racing aptitude, distance, female family, genetic considerations, commercial appeal, stud fee and breeder objectives all remain part of the decision.

AEI and Comparable Index should not replace those considerations.

They can, however, add another level of statistical analysis.

For a Quarter Horse mare owner studying two stallions with similar progeny earnings, similar stakes production and similar stud fees, the AEI-to-CI relationship could provide valuable additional information.

Instead of asking only:

"How much have his foals earned?"

the breeder can begin asking:

"How well have his foals performed compared with the average—and how does that performance compare with the quality of mares he has been given?"

That is a considerably more sophisticated question.

And as more statistical information becomes readily available to Quarter Horse breeders, Average Earnings Index and Comparable Index may become increasingly useful tools for evaluating the performance of today's stallions.

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