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How to calculate the probability of an event NOT happening

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How to calculate the probability of an event NOT happening: Overview

How to calculate the probability of an event NOT happening

What’s the probability of an event NOT happening?


Probability questions are often word problems that appear more difficult than they actually are. The trick is to identify the type of probability problem you have. This article will tell you how to find the probability of a simple event not happening. You can also check out the > Probability Index for more question types.

How to calculate the probability of an event NOT happening: Steps

  • Step 1: Choose one of the following sample questions below that most closely matches your problem and go to the step indicated. Pay particular attention to the bold words when selecting.
    1. In a certain year, 12,942 construction workers were injured by falling debris. The top six cities for injuries  were: New York  141,  Houston 219, Miami 112, Denver 140, and San Francisco 110. If an injured construction worker is selected at random, what is the probability that the selected worker will have been injured in a city other than Houston, Miami, or Denver? If your problem looks like this, go to step 2.
    2. If the probability that a major hurricane will hit Florida this year is 0.49, what is the probability of a major hurricane not hitting Florida this year? If your problem looks like this, go to step 5.
    3. Twenty seven percent of American adults support Universal Health Care. If an American adult is chosen at random, what is the probability they will not support Universal Health Care? If your problem looks like this, go to step 6.
  • Step 2: Add up the given cities.
    Houston (219) + Miami (112) + Denver (140) = 471
  • Step 3: Divide the number you found in step 2 by the total number of people or items given. In this case, we were told that 12,942 construction workers were injured. So:
    471 / 12,942 = .0364.
  • Step 4: Subtract the number you found in step 3 from 1 (that is, the number one, not step 1).
    1 – .0364 = 0.9636(rounded to four decimal places).You’re done!
  • Step 5: Subtract the “not” probability from 1.
    1 – 0.49 = 0.51. That’s it!
  • Step 6: Convert the percentage to a decimal.
    Twenty seven percent = 0.27
  • Step 7: Subtract the number in step 6 from 1 (that is, the number one not step 1!).
    1 – 0.27 = 0.73.That’s it–you’re done!

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5 thoughts on “How to calculate the probability of an event NOT happening

  1. FHSV

    The links from both the STA2023 FAQ and the website’s Post Index for “How to find the probability of an event NOT happening” are both disabled/broken. I was able to locate the page after ‘Googling’ it.

  2. James

    1 – .0364 = 0.9936 (rounded to four decimal places). ???:

    I am really math dumb, but when I subtract .0364 from 1 I end up with 0.9636 and NOT 0.9936 as indicated am I doing something wrong here?

  3. James

    English determinate agreement, just for information:
    If AN injured construction worker is selected at random,