How field sizes can affect win percentages

November 15th, 2023

Many factors can influence the win percentages recorded by jockeys and trainers, both overall and in specific types of races. Ability and opportunity are two of the biggest factors, but another is field size—the number of horses entered in a race.

Picture it this way: in a race with five horses, each horse theoretically has a 20% chance at winning. That’s double the 10% win chance afforded to each entrant in a 10-horse field.

Obviously not all horses are equally fast, so in reality some of the horses in a 10-horse field will have better than a 10% chance at victory, while others will have less than a 10% chance. But across thousands of races, any given horse in a 10-horse field is going to win less often than any given horse in a five-horse field.

It’s important to keep this in mind when analyzing jockey and trainer stats, because the influence of field size should be considered when deciding what constitutes a positive or negative win percentage. If a jockey wins 20% of the time on dirt and 15% of the time on turf, you might assume the jockey is more skilled riding dirt races than turf races.

But turf races tend to feature larger fields than dirt races. If the jockey has been riding dirt races with an average field size of seven horses, then all else being equal the jockey should win at a 14% rate on dirt. Whereas if the jockey is riding turf races where the average field size is 10 horses, the jockey’s win rate on grass should be only 10%.

Here’s another example: suppose Trainer A races at a track where the average field size is nine horses, while Trainer B races at a track where the average field size is six horses. All else being equal, Trainer A’s win percentage should be 11%, while Trainer B’s win percentage should be 17%. This doesn’t necessarily mean Trainer B is better than Trainer A, or that Trainer B has better horses; it simply means Trainer A is competing in races that are harder to win.

These are some extreme examples, but they’re worth remembering. You’ll still encounter circumstances where a jockey does indeed ride better on dirt than turf (or vice versa), but realizing how field sizes can skew win percentages will help prevent you from jumping to incorrect conclusions based off win percentages without context.