Parlays & Correlation

Parlays are marketed as the path to a huge payout, and they can be fun - but the math usually favors the book heavily. Understanding correlation is the key to knowing when a parlay is a trap and when it's an edge.

Why standard parlays are expensive

A parlay's payout is calculated as if every leg is independent, and the vig compounds across all of them. A two-leg parlay stacks two margins; a five-leg parlay stacks five. That's why random parlays of favorites are among the worst-value bets on the board - the hold can exceed 20-30%.

What correlation means

Two outcomes are correlated when one happening changes the odds of the other. If a quarterback throws for 350 yards, his team is more likely to score a lot of points - so "QB over 300 passing yards" and "team over 27.5 points" tend to happen together. Books price legs as if they're independent, so correlated legs can be mispriced.

Same-game parlays and correlation

Same-game parlays (SGPs) let you combine legs from one game. Books try to adjust for obvious correlations, but they can't catch everything. When you can identify legs that are positively correlated but priced as independent, an SGP can actually carry *positive* EV - the rare case where a parlay favors the bettor.

The practical takeaway

• Avoid random multi-leg parlays - the compounding vig is brutal.
• Never parlay uncorrelated favorites for the payout; it's the book's favorite bet.
• Look for positive correlation the book hasn't fully priced when you do play SGPs.
• Beware negative correlation - some legs make each other less likely, quietly worsening your odds.