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 Description  
 Certain Event:  An event with a probability of 1.
Note that an event that cannot possibly happen has a probability of zero. If there is a chance that an event will happen, then, its probability is between zero and 1.

Examples of Events that are Certain:
  • If it is Thursday, the probability that tomorrow is Friday is certain, and therefore the probability is 1.
  • If you are sixteen, the probability of you turning seventeen on your next birthday is 1. This is a certain event.


Examples of Events That Are Not Certain:

  • The probability that tomorrow is Friday if today is Monday is 0.
  • The probability that a coin you toss will land on tails is .50.
  • The probability that the groundhog will see its shadow on Groundhog Day.