Probability

What is Probability?

Probability is the chance of an outcome occurring, expressed as a fraction, a decimal or a percentage.

Why it matters

Probability converts odds into a number you can compare against your own view of the outcome.

Example

One number on a European roulette wheel is 1 in 37, which is 2.7 percent. Decimal odds of 4.00 imply a 25 percent chance, calculated as 1 divided by 4.

Common misunderstandings

• Independent events have no memory. A wheel that has landed red ten times running is exactly as likely to land red on the next spin. Believing otherwise is the gambler fallacy.

Frequently asked questions

How do I convert odds to probability?

Divide 1 by the decimal odds. Odds of 2.50 imply 40 percent. Multiply by 100 to read it as a percentage.

Do past results change the next result?

No. Roulette wheels, dice and random number generators produce independent results. The previous outcome carries no information about the next one.

Related terms

• Odds
• Margin
• House Edge
• Expected Value (EV)