Fair Odds and De-vigging
Sportsbooks usually price both sides so their implied probabilities add to more than 100%. De-vigging removes that excess before the market is used as a fair estimate.
Why both sides matter
A single price does not reveal the market's margin. A reliable calculation pairs opposite outcomes at the same line and removes the excess probability from the pair.
A simple example
If both sides are -110, each implies about 52.4%. After normalizing the two probabilities back to 100%, each side is estimated at 50%, or fair odds of +100.
More sources can improve the estimate
Independent prices can make a fair estimate more stable, but only when the event, market, participant, period and line are matched correctly. An unrelated alternate line is not interchangeable.
Fair does not mean certain
A fair price is an estimate built from available market evidence. It can move as books update, information changes or liquidity improves.