Definition
Probability
Probability is a number from 0 to 1 that measures how likely an event is.
What it means
Probability turns chance into a number you can compare. It may be written as a fraction, decimal, or percent. The number must come from a stated model or from repeated evidence; do not assume all outcomes are equally likely unless the model says so.
Remember it
Zero says cannot; one says certain; numbers between measure the chance.
Example
A box has two yellow cubes and three purple cubes, all equally easy to draw. The probability of drawing yellow is 2/5 = 0.4 = 40%.
Watch for
Counting two colours is not enough to assign 1/2 to each colour when there are different numbers of cubes or the drawing method is biased.
Common mix-up
Probability describes the model or long-run chance; it does not predict the exact next outcome.
Exact meaning
In a stated probability model, probability assigns an event a number from 0 to 1. A value of 0 marks an impossible event, 1 marks a certain event, and larger values mean greater likelihood.