Mathematics (Core)Exam code: 0580

Basic Probability

Probability

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Measuring likelihood

  • Every probability is a number from 0 to 1, where 0 means impossible and 1 means certain
  • Halfway, at 0.5, the outcome is as likely to happen as not
  • Answers may be given as a fraction, a decimal or a percentage, and all three are accepted, but a ratio such as 7 : 20 or words such as "7 out of 20" can lose the mark
  • Where the question does not specify, a fraction is usually safest, since it stays exact
  • A probability outside 0 to 1 is always an error, so check any answer above 1 or below 0
A probability scale running from 0 to 1 with words underneath: impossible at 0, unlikely between 0 and 0.5, even chance at 0.5, likely between 0.5 and 1, and certain at 1, with a flying pig at the impossible end, a dice above the unlikely side, a coin at even chance, a spinner that is three-quarters green above the likely side, and a moon beside the sun at the certain end
Source: Basic probability by Save My Exams

The words that carry meaning

  • An experiment is repeated to produce results, and each repeat is a trial
  • An outcome is one possible result; an event may be a single outcome or several of them taken together
  • The sample space collects every possible outcome, and is usually set out either as a list or in a table
  • Something is fair when every outcome is equally likely, and biased when they are not
  • Picking at random means every item has the same chance of being chosen

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