Odds Drift: Meaning, Calculation and Interpretation

Worked drift calculation

If decimal odds move from 1.80 to 2.05, the price has drifted.

• Price change = 2.05 - 1.80 = 0.25.
• Relative price change = (2.05 / 1.80) - 1 = 0.13889, or 13.889%.
• Earlier raw reciprocal = 1 / 1.80 = 0.55556, or 55.556%.
• Later raw reciprocal = 1 / 2.05 = 0.48780, or 48.780%.
• Raw probability change = 48.780% - 55.556% = -6.776 percentage points.

Do not call the move a 13.889-percentage-point probability decline. Price percentage and probability-point change are different measures.

Capture enough context

Store source, event, market, selection, settlement period, both timestamps, complete market prices, availability and market state. Betfair's historical data specification provides a first-party example of timestamp, price, volume and status fields required to reconstruct a market path.

A drift is not automatically a bargain

Suppose a model originally estimated p = 0.52. At 1.80, EV = (0.52 * 1.80) - 1 = -0.064 units. At 2.05, EV = (0.52 * 2.05) - 1 = 0.066 units.

The later price appears positive only if 0.52 remains defensible after whatever changed. If new information lowers the estimate to 0.47, EV = (0.47 * 2.05) - 1 = -0.0365 units. Re-run the model with information available at the later timestamp rather than carrying forward a stale probability; calibration guidance explains why the updated probability still needs later reliability checks.

Possible causes remain hypotheses

News, lineup information, reference-price movement, margin changes, liabilities or unrelated trading decisions can coincide with drift. A single price series cannot identify the cause. Compare publication times and multiple sources, and describe the result as association unless stronger evidence exists.

Reader workflow

The sequence below is a local decision rule built around expected-value arithmetic and timestamped market data:

1. Verify that both quotes refer to the same settled event.
2. Recalculate the complete market overround at each timestamp.
3. Update the model only with information genuinely available then.
4. Evaluate the later executable price, including costs.
5. Record a pass when the reason for the move or probability sensitivity is unresolved.

Compare complete market snapshots

A selection can drift while the total market margin also changes. Capture every mutually exclusive outcome at both timestamps and calculate both booksums. Then compare raw and de-margined movement using one method. This distinguishes a selection-specific share change from a broad repricing of the whole market.

Also check multiple sources. If one operator moves from 1.80 to 2.05 while a reference market remains near 1.82, the observation may be source-specific. If many independent sources move in a narrow interval, the shared movement is stronger evidence of a market-wide update, but its cause still requires separate evidence. Preserve non-movers and unavailable quotes rather than reporting only sources that changed.

Next step

Use Odds Shortening for the next part of this topic.

Continue learning

• Next guide: Odds Shortening
• Related guide: What Is Value Betting? Probability, Price and Uncertainty

Assumptions and limitations

The example is illustrative. Odds can drift at one source while shortening at another, and displayed quotes may not be accepted. This page defines drift; it does not recommend betting against market movement.