What the closing line is
The closing line is the last price a sportsbook offers before a game starts. By then the market has absorbed the injury news, the lineups, the weather and the money of every bettor with an opinion. That makes it the most informed price the market ever produces for that game.
It is not always right — nothing is — but across thousands of games, closing prices are remarkably well calibrated. That is why they make a useful yardstick.
What beating it means
Closing line value, or CLV, measures whether the price you took was better than the closing price. If you backed a team at +120 and the game closed at +100, you got a better payout than the market's final, best-informed view. You beat the close.
In probability terms, +120 implies 45.5% and +100 implies 50%. You bought a win chance that the closing market valued about 4.5 percentage points higher than your price did.
Why it matters so much
Wins and losses are noisy. A good bet at 55% loses 45 times out of 100, so a short run of results says little about whether the bets were good. Closing line value is far less noisy: each bet tells you immediately whether you beat the most informed price, before the game is even played.
Over time, a process that consistently gets better prices than the close has usually found real information. A process that consistently gets worse prices usually has not, whatever its recent results say.
Measuring it fairly
Compare like with like: the same market, the same side, and ideally no-vig closing prices so the sportsbook margin does not distort the comparison. Record the price you actually took at the time you took it — not the best price seen afterwards.
SharpDeck records each forecast with the best price on offer when it was published, and never updates it when a later quote arrives. That is what makes it possible to compare a forecast with the close honestly.
What CLV does not tell you
Positive CLV is a strong sign, not a promise. Markets can move for reasons that have nothing to do with the true probability, and a small sample of closing prices can mislead just like a small sample of wins. Read it together with the long-run record, never instead of it.
SharpDeck field notes. Product explanations and worked examples, not betting recommendations.