|>>|| No. 4994
>Does that mean 50/1 are good odds? Or risky?
50/1 implies that the chance of that outcome is approximately 1.96%. Odds of 2/1 imply a probability of 33.33% and 1/1 ("evens" in bookmaking parlance) implies a 50% chance. Odds with a low implied probability are called "long odds" and odds with a high implied probability are called "short odds". Commentators will often talk about the odds lengthening or shortening, as bookmakers adjust their odds based on new information or changes in the betting. Odds shorter than evens are called "odds on". Odds of 1/4 imply a probability of 80%.
The job of a bookmaker is to set odds slightly longer than the actual probabilities, such that the probabilities implied by the odds for all of the possible outcomes sum to greater than 100%. This is called "over-round" and is how bookmakers turn a profit. In the long run, the over-round ensures that the bookmaker will pay out less than he takes in. The over-round is equivalent to the house advantage in casino games like roulette.
From the gamblers perspective, "good odds" are where the bookmaker has over-priced an outcome, providing the gambler with an opportunity to profit. If a gambler believes that a horse has a 1 in 4 chance of winning, then odds longer than 3/1 would be favourable for him.
This traditional notation (fractional odds) is inconvenient for giving very precise odds or performing calculations, so modern gamblers tend to favour decimal odds. Decimal odds represent the total return on a given stake, so a bet of £1 at odds of 2.00 will return £2, a bet of £100 at 1.02 will return £102, a bet of £5 at odds of 10.00 will return £50 and so on.