Definition
Correlation measures how strongly two outcomes move together. Positive correlation means they tend to happen together. Negative correlation means one outcome tends to make the other less likely. Zero correlation means the outcomes do not move together in a useful pattern.
In betting math, correlation is often measured with the correlation coefficient:
r = covariance(X, Y) / (standard deviation of X × standard deviation of Y)
The value ranges from -1 to +1. A value near +1 means strong positive correlation. A value near -1 means strong negative correlation.
Worked Example
A bettor likes a football team at -110 and also likes that team’s quarterback to go over 250.5 passing yards at -110.
Each bet risks $110 to win $100. If the team often wins when the quarterback throws for a high yardage total, those bets are positively correlated. The bettor is not making two fully separate bets; both depend partly on the same game script.
If the team wins mostly through rushing and clock control, the team moneyline and quarterback passing over could be negatively correlated.
Why It Matters
Correlation helps a bettor avoid overstating diversification across bets that are driven by the same outcome. It also matters in parlays, props, DFS lineups, and portfolio staking because linked outcomes change true risk and payout value.