What is line movement?
Line movement refers to the change in odds offered by a bookmaker between the time a market is first opened and the time it is closed. The odds at the moment the market opens are called the opening odds, and the odds at the moment just before the event starts are called the closing odds. The trajectory of changes between these two points is the line movement.
Line movement is not random. It reflects the continuous process by which the bookmaker adjusts its estimation of the probabilities of the event, incorporating new information as it becomes available. Understanding why and how lines move is essential for evaluating whether the odds available at any given moment represent a favorable or unfavorable opportunity.
Why odds change
Odds change for several reasons, which can be broadly classified into two categories: information-driven adjustments and market-driven adjustments.
Information-driven adjustments occur when new facts about the event become known. Examples include injuries to key players, changes in team lineups, weather conditions, or tactical decisions announced by coaches. When such information becomes public, the bookmaker recalculates its probability estimates and adjusts the odds accordingly.
Market-driven adjustments occur in response to the distribution of wagers placed by players. When a disproportionate amount of money is wagered on one option, the bookmaker may lower the odd for that option and raise the odds for the other options. This serves two purposes: it reduces the bookmaker's risk exposure to a lopsided payout, and it reflects the collective information embedded in the players' decisions — particularly when the wagers come from players with a demonstrated record of accurate predictions.
Sharp and recreational players
Bookmakers distinguish between two broad categories of players based on their historical accuracy. Sharp players, sometimes called informed players, are those whose bets have historically been profitable, indicating that their probability estimates tend to be more accurate than the bookmaker's initial estimates. Recreational players are those whose bets have not demonstrated consistent accuracy.
The wagers of sharp players carry disproportionate influence on line movement. When a sharp player places a large wager on one option, the bookmaker interprets this as a signal that its current odds may be mispriced, and adjusts accordingly. This adjustment may occur even if the total monetary volume on that option is small relative to the recreational volume on the other side.
A rapid and significant movement in the odds, triggered by the concentrated action of sharp players, is sometimes called a steam move. Steam moves often occur shortly after the market opens, when sharp players identify the largest discrepancies between the opening odds and their own probability estimates.
Opening odds and closing odds
Opening odds represent the bookmaker's initial estimation of the probabilities, produced before any wagers have been placed. They are based on the bookmaker's internal models, expert analysis, historical data, and the odds offered by competing bookmakers.
Closing odds represent the final estimation, refined by all the information and market activity that has accumulated between the opening and the start of the event. Because closing odds incorporate the maximum amount of available information — including the aggregated estimations of all players who have wagered — they tend to be the most accurate prediction of the actual probabilities of the event.
Empirical research has consistently shown that closing odds are well-calibrated predictors: events whose closing odds imply a probability of, say, 40%, tend to occur approximately 40% of the time across large samples.
Closing line value
Closing line value is the difference between the odd at which a player placed his bet and the closing odd for the same option. If a player consistently places bets at odds higher than the closing odds, he is said to have positive closing line value. This means that, by the time all available information has been incorporated into the market, the odds have moved in the direction the player predicted — indicating that his probability estimates were, on average, more accurate than the market's initial estimates.
Closing line value is widely regarded as the most reliable indicator of long-term profitability. A player who consistently achieves positive closing line value is expected to be profitable over time, even if short-term results are negative due to variance. Conversely, a player who consistently bets at odds worse than the closing line is expected to lose money in the long term, even if he experiences short-term winning streaks.
For example, if a player bets on an option at a decimal odd of 2.30, and the closing odd for the same option is 2.10, the player has obtained positive closing line value: the market ultimately agreed that this option was more likely than its initial odds suggested, and the player secured his bet at a more favorable price.
Practical implications
Understanding line movement allows a player to make more informed decisions about when to place a bet. If the player expects the line to move in a particular direction — for example, because he has identified information that the market has not yet incorporated — he benefits from placing his bet before the movement occurs, thus securing a higher odd.
Conversely, observing line movement can itself serve as information. If the odds for an option are shortening (the odd is decreasing, implying a higher probability), it may indicate that sharp players or significant new information favor that option. If the odds are drifting (the odd is increasing, implying a lower probability), it may suggest the opposite.
However, line movement alone does not determine whether a bet has positive expected value. The critical factor remains the relationship between the actual probability of the event and the odd at which the bet is placed. Line movement is a valuable signal, but it must be interpreted within the broader framework of probability estimation and expected value calculation.