Rich – good stuff, you’re absolutely on the right track in determining whether there really is a home court advantage.

When you compare the percentage of winners to the percentage of runners as you’ve done, you’re measuring what’s called the “impact value”. In its raw form, it’s a good metric, although it can be prone to misleading results because it doesn’t take into account each horse’s actual probability of winning (odds). What's even better to use is what’s called an odds-based impact value: Odds-Based Impact Value = Actual Win% / Expected Win %

I posted about just this topic a few years ago in which I included a link to a great little article written by Gordon Pine that’s definitely worth checking out:

[url=https://www.thorograph.com/phorum/read.php?1,83098,83388,quote=1]Impact Values[/url]

As you can see from the results you generated, which on the surface look to show a rather pronounced California bias, impact values (and particularly odd-based impact values) are a powerful tool to help determine if a particular factor or angle is winning at an outsized rate. I use the runstyle and post bias impact factors listed at the back of each race’s Brisnet PP’s regularly to see if there are any biases associated with that particular track profile (even though they’re of the inferior raw form, they’re still quite useful).