When a bookmaker prices a football match, it does not offer you its honest opinion of each outcome. It offers you a price with a markup baked in, called the margin or overround. That margin is how bookmakers stay in business even when their predictions are wrong, and it is the first obstacle any bettor must understand before judging whether a price is good value.

The good news is that the margin is not hidden. It is arithmetic, and you can strip it out of any set of odds in under a minute. Once you do, you stop comparing prices at face value and start comparing the bookmaker's true implied probabilities against your own judgement. That single habit separates a casual punter from a disciplined one.