According to Pitman, “The probability of an event is a measure of the likelihood or chance that the event occurs, on a scale from 0 to 1” (Pitman, 1993, p. 1); as this simple definition suggests probability is not only applicable to advanced science and mathematics, but also to ordinary, everyday life. This can be seen by examining the rules of probability as they occur on a daily basis.
One rule of probability is the rule of subtraction: “P(A) = 1 – P(A’)” (Stat Trek, 2016, n.p.), or the idea that the probability of an event occurring equals 1 minus the probability of the event not occurring. An example of this might be determining the probability of whether school will be delayed because of snow. If the probability of this event occurring is 0.70 based on the school snow policy and the current weather forecast, then the rule of subtraction states that the probability of school not being delayed must equal 1 minus 0.70, or 0.30. This is the sort of calculation many people make roughly on a day-to-day basis, as a means of deciding how they should act to prepare for the near future.

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Another rule of probability is the rule of addition: “P(A ∪ B) = P(A) + P(B) – P(A ∩ B))” (Stat Trek, 2016, n.p.), or in other words, the probability of one of several events occurring equals the probability of the first event plus the probability of the second event, minus the probability of both events occurring. So, I might use this to calculate the probability of the canteen serving either peas, or carrots, or both for lunch. If the probability of carrots being served is 0.30, the probability of peas being served is 0.40, and the probability of being served both is 0.10, then P(Carrots ∪ Peas) = 0.30 + 0.40 – 0.10 = 0.60, or the possibility of being served either peas, or carrots, or both is 0.60. While not many people bother to calculate this kind of probability formally, many people make unconscious calculations about this type of probability all the time, and base their decisions on them.

As these examples demonstrate, therefore, probability can be used to answer some of the questions that make up the fabric of everyday life.

  • Pitman, J. (1993). Probability. New York: Springer Science-Business Media Inc.
  • Stat Trek (2016). “Rules of Probability.” Retrieved from: >