Overround

Add up the implied probabilities of every outcome in a market and you'd expect 100%. It never is, and the gap is the whole game.

Convert each price to its implied probability (1 divided by the odds) and sum them. A two-way market priced 1.91 / 1.91 comes to about 105%. That extra 5% is the overround, the same thing as the vig, just viewed from the market's side rather than a single bet's.

Comparing overrounds tells you instantly which book is tighter. A match priced at 102% total is a sharper, fairer market than the same one at 109%, and over a season that difference is most of your profit or loss.