Are betting odds different in different countries: the basics
Behind the counter, "Are betting odds different in different countries?" traces back to an estimate. Every price a bookmaker offers begins as an estimate of probability and ends as a number with a margin added. The estimate comes from data, models and traders; the margin comes from the business. Knowing how the two are combined explains why prices move, why they differ between books, and why the chances they imply never add up to exactly a whole.
Do betting odds add up to one? The implied probabilities of every outcome in a market would, if the prices were fair. In practice they add up to more, often one hundred and four or one hundred and five per cent on a two way market and more on markets with many outcomes. That excess is the overround, the book's margin, and it is the reason a bettor backing every outcome loses money with certainty.
Are betting odds accurate? As forecasts, closing prices are well calibrated: outcomes priced at a fifty per cent chance happen close to half the time across many events. As prices, they are deliberately a little short of fair. Betting odds are different in different countries mostly in notation and in margin: decimal, fractional and American formats show the same chance, while books serving smaller markets tend to add a wider cut.
Betting odds implied probability is the chance a price suggests, before any judgement about whether it is right. For decimal odds it is one divided by the price. To convert American odds to a percentage, a positive number gives one hundred divided by the number plus one hundred, and a negative number gives the number, without its sign, divided by itself plus one hundred. A price of +150 implies forty per cent, and a price of -200 implies close to sixty seven.
Answers in brief
What makes odds move before an event starts?
News such as injuries, lineups or weather moves them, and so does money, with stakes from sharp bettors shifting prices faster than casual ones.
Why does a price shorten when money piles on one side?
The book shortens the side taking most stakes and lengthens the other, so it is never heavily exposed to a single result.
Do odds differ from one country to another?
Mostly in notation and margin: decimal, fractional and American formats express the same chance, while books serving smaller markets often take a wider cut.
What is the overround in a betting market?
The amount by which implied chances across all outcomes exceed one hundred per cent, often around four or five per cent on a two way market.