Key numbers are the most common margins of victory in the NFL and NBA. Due to the nature of scoring in the NFL, the key numbers are much more obvious and apparent.
Far and away, the biggest key number in football is 3. Games that are tied late (or that eventually go into overtime) are very often settled with a game-winning field goal as teams often play much more conservatively and go for the field goal instead of the touchdown once they get within scoring range. NFL games end by a 3-point margin about 15% of the time.
About 7% of NFL games end in the NFL’s other most common key number: 7. This makes obvious sense as well, considering touchdowns are worth seven points. If a team is up by seven near the end of the game, their opponents will be going for a touchdown, since a field goal will do them no good. If they fail to score that touchdown, the winning team can simply run out the clock and secure their 7-point win.
Sportsbooks are hesitant to move spreads away from key numbers, especially the extremely common 3-point spread. Instead, you often see the price adjusted as an alternative; for example, if San Francisco -3.0 -110 is receiving a lot more action than Seattle +3.0 -110, you could see the prices move to San Francisco -3.0 -120 and Seattle +3.0 +100.
Sportsbooks don’t want to move off of a key number like the 3-point spread, because if they move San Francisco to -3.5 and get a lot of action on St. Louis +3.5, a 3-point San Francisco win would result in a major loss for the book. Action on San Francisco -3.0 would be a push, while they’d have to pay out all of the St. Louis +3.5 action.
NBA key numbers are harder to diagnose but the most common one is 7, as a little more than 7% of NBA games end by that margin. This is because of fouls at the end of the game; once the gap is at seven points, it is a three-possession game, and the losing team concedes.