Understanding the Exact Score (E) and Similar Score (CSI)
When Stallion Match returns a list of similar stakes winners, each one carries two scores: Exact Score (E) and Similar Score (CSI). Both compare that stakes winner's pedigree against the mating you searched, but they measure different kinds of similarity.
The key difference
Exact Score counts ancestors appearing in the same pedigree position in both.
Similar Score counts shared ancestry across the corresponding side, whether or not the positions match.
Sire side is compared with sire side, and dam side with dam side. An ancestor appearing on the sire side of one pedigree and the dam side of the other is not a match, because that describes a different genetic relationship.
Exact Score (E)
The Exact Score measures how closely the two pedigree structures line up. An ancestor counts only when it occupies the corresponding position in both pedigrees.
Example. Danehill is the sire's damsire in Pedigree A. Danehill is also the sire's damsire in Pedigree B. That is an exact match. If Danehill appeared somewhere else on the sire side of Pedigree B it would not count towards the Exact Score, although it could still contribute to the Similar Score.
A higher Exact Score means more ancestors sit in identical positions, so the two pedigrees are built the same way rather than merely containing the same horses.
Similar Score (CSI)
The Similar Score measures how much common ancestry the two pedigrees share across both sides, within five generations. Position does not matter, only the side.
Example. Danehill appears on the sire side of Pedigree A. Danehill also appears on the sire side of Pedigree B, in a different position. That contributes to the Similar Score even though it is not an exact match.
CSI is displayed as a percentage with a count in brackets, for example 70%(22), so you can see the proportion and the number of shared ancestors behind it at the same time.
How the two scores are counted
Similar Score. The distinct ancestors shared between the two pedigrees, expressed as a percentage of 31, which is the number of ancestors on one side of a five generation pedigree. A CSI of 70%(22) means 22 distinct shared ancestors out of a possible 31.
Counting distinct ancestors matters. An ancestor appearing several times through duplication is counted once, so a heavily linebred pedigree does not inflate its own score.
Exact Score. A count of the ancestors sitting in exactly the same position in both pedigrees, shown by the dark green shading in the side-by-side comparison. The maximum is 62, being 31 on the sire side and 31 on the dam side.
That is why the two numbers use different scales. A full sibling matches every position on both sides, so it returns an Exact Score of 62 alongside a Similar Score of 100%(31).
Reading the two together
The scores are not a broad and narrow version of the same thing. They are counted differently and answer different questions, so a stakes winner can score well on one and poorly on the other.
High CSI, low E. The two pedigrees contain much of the same material arranged differently. Real shared ancestry, looser structural resemblance.
High E, lower CSI. The parts that match, match precisely, but less of the wider pedigree is shared.
Both high. The strongest resemblance, and the rarest.
Two examples from a single result list show the range. Two stakes winners can both show CSI 70%(22) while one has an Exact Score of 31 and the other has 0. Same shared ancestry, completely different structure.
What the top of the range looks like
Matching Siyouni with Soustraction returns Algebre at the top of the list, with an Exact Score of 62 and a Similar Score of 100%(31).
Algebre is by Siyouni out of Soustraction, so he is a full sibling to the foal the mating would produce. Every ancestor in both pedigrees is the same horse in the same position, which is why the comparison shows no gaps.
That is the ceiling, and it is worth seeing once so you know what the numbers mean at the top of the range. Most useful results sit well below it.
The side-by-side comparison
Click the arrow at the end of any row to open Side By Side Comparison. Your hypothetical mating appears on the left and the selected stakes winner on the right, with matching ancestors highlighted.
This is where the scores become concrete. A number tells you a resemblance exists. The comparison shows you which ancestors account for it and where they sit, so you can judge whether the match runs through parts of the pedigree you care about or through distant common stock.
Full screen enlarges the comparison, and Legend explains the highlighting.
Beneath the pedigrees, the stakes winner's own race record is listed with the type, race, year, location, distance and track, so you can see what the horse actually achieved rather than only that it reached stakes level.
Where else these scores appear
The same two measures drive the ratings. A pedigree earns a 20/20 Match when multiple stakes winners share at least a 65% match with it across five generations, which is a Similar Score threshold.
The Impact Profile's Compare Pedigrees tab applies the same idea between any two horses you choose, reporting Similarity Score, Shared Ancestors, Exact Score and Exact Ancestors. The labels differ, the measures are the same.