Opening and closing odds show two points in a market's history. The opening quote establishes an early reference; the closing quote records the final available pre-match price under the data provider's collection rule. The difference shows a price change. It does not, by itself, explain who moved the market or why.
Shortening and drifting
When decimal odds fall from 2.50 to 2.20, the price has shortened. The raw implied probability rises from 40.00% to 45.45%. When odds rise from 2.20 to 2.50, the price has drifted and the raw implied probability falls.
The first move is a 12% price decrease: (2.20 ÷ 2.50 − 1) × 100. The implied probability increases by 5.45 percentage points. Those figures measure different things; neither should be presented simply as a 12% increase in the chance of winning.
Check the other side of the market
If a home price shortens, inspect the draw and away prices too. A change in the bookmaker's overround can affect raw implied probabilities even when the underlying balance of the market changes little. For totals and handicaps, check whether the line moved as well as the price.
Over 2.5 goals at 1.90 and over 3.0 goals at 1.90 have the same quoted odds but different settlement outcomes. Plotting them as an unchanged price would hide a material market change.
A timestamp makes the comparison meaningful
Record the timezone, event start and collection cut-off. Distinguish an actual quoted opening price from the first record a provider happened to capture. Exclude in-play prices from a pre-match comparison and document postponed fixtures whose original start time changed.
OddsTips guide to opening and closing odds explains why source, observation time and market context belong together. A sparse archive of two snapshots cannot reconstruct every movement between them.
Use the close as a reference, with limits
A later price may incorporate team news, market activity and revisions to earlier expectations. A closing quote can still be stale, lightly traded or affected by its own margin. There is no basis for calling every price drop informed money.
Our closing line value article shows how to compare an earlier price with a closing benchmark. For comparisons between bookmakers at the same point in time, start with odds comparison.