"This year’s Guineas winner Dawn Approach has some illustrious hoofprints to fill, but this “myth” provides encouragement that he can do so, for it is not really a myth at all..."
Simon Rowlands analyses the recognised Derby Trials in an attempt to discover which provides the best pointers for Epsom...
This saying is not as popular as it once was - indeed, it took no small amount of Googling to relocate it - but the idea that the best trial for the second colts' classic is the first colts' classic has a certain ring of truth to it.
Against that, the Guineas is run at a mile at Newmarket, while the Derby is run at just over a mile and a half at Epsom. It is asking something of a young horse to excel at both in the space of less than a month.
As with many a "myth", its veracity depends in no small part on how you look at the evidence. Judged on winners alone, the Guineas is equal on four with the Dante for three-year-old "trials" this century, though in New Approach's case he ran in the Irish Guineas between defeat at Newmarket and success at Epsom.
If we take a horse's most recent run prior to the Derby as its definitive trial for that race, the Dante is alone at the top, but from 28 representatives this century against 18 for the Guineas. The strike-rate for horses going to Epsom straight from the Guineas is higher than the same for the Dante, in other words.
Regular readers of this blog will know that wins, and wins alone, can be a crude measure, and that strike-rates are often little better: the former disregards the wealth of nuanced information from the majority of horses that do not win, while the latter fails to allow for opportunity, such as reflected in field size.
One way to look at this issue is to consider the percentage of rivals beaten by horses in a given trial and then the percentage of rivals beaten by that cohort in the Derby itself. Percentage of rivals beaten reflects, for instance, that it is better to finish second of 15 than second of five.
There is one slight drawback with this approach, and that is our old friend Regression To The Mean, which dictates that winners of trials will be punished compared to horses out the back. If you have beaten 100% of your rivals in a trial, you can at best match that in the Derby itself; conversely, if you have finished plum last in a trial you can do no worse (and may do better) in the big race itself. There is a tendency for extremes to head towards the middle.
An adjustment for this phenomenon alters the figures somewhat, but not the ranking, and has the Guineas as top on a factor of 0.78 (a horse that had beaten 50% of its rivals in the Guineas could be expected to beat 39% of its rivals in the Derby), followed by Leopardstown's Derrinstown Stud Derby Trial on 0.76, the Dante on 0.74 and the Chester Vase on 0.68.
One interesting consequence of this is that it implies finishing second in this year's Guineas (beating 91.7% of rivals, with an expectation of beating 71.5% at Epsom) would be better than winning the Chester Vase (expectation of beating 68% of rivals at Epsom).
The figures for all trials (latest run only) which produced five or more runners in the Derby this century are shown here: 
By this measure the Guineas is indeed the best trial for the Derby, but only just. By other measures, it could be argued that it is not, but only just.
This year's Guineas winner Dawn Approach has some illustrious hoofprints to fill, but this "myth" provides encouragement that he can do so, for it is not really a myth at all.
While the figures quoted are primarily descriptive of past events, I will, for interest, calculate expected % rivals beaten when all the named trials have been run and post those expectations in the comments section below.
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