Probability: Probability is used to calculate the likelihood that something will happen; as a ratio, the number of desired outcomes to the total number of possible outcomes, which is expressed as a number between 0 to 1, where 0 implies "impossible" and 1 implies "certain" or "sure". This is also expressed as the "chance" of occurrence of the event. In this, case the scale is from 0% to 100%. Examples: 1) "It is unlikely to rain tomorrow". 2) As a number, probability is from 0 (impossible) to 1 (certain). Impossible Unlikely Even Chance Likely Certain O 1-in-6 Chance 4-in-5 Chance . Probabilities are expressed as fractions (and sometimes in decimal form)