How to Calculate a P-Value

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According to The Cartoon Guide to Statistics, p-value is a probability statement which answers the question: If the Null Hypothesis is true, then what is the probability of observing test statistics at least as extreme as the one observed. The Null Hypothesis, is usually that the observations are the result of chance. The Alternative Hypothesis is that there is a real effect, that the observations are the result of this real effect, plus chance variation.

    State the Null Hypothesis and the alternative to it.

    Select the level of significance.

    Select the test statistic.

    Determine the appropriate sampling distribution for the test statistic.

    Find the sample value of the test statistic and accept the null hypothesis if the value of the test statistic lies within the acceptance region. You now have the p-value.


    • According to Statistics in a nutshell, p-values are commonly reported in most research results involving statistical calculations. (145) Use p-values to guild your intuition to make an informed decision.


    • According to Statistics in a Nutshell, beware of any piece of research that attempts to prove a theory by a single experiment.


  • Statistics in a Nutshell, Sarah Boslaugh, Paul Andrew Watters, 2008
  • The Cartoon Guide To Statistics, Laryy Gonick, Woolcott Smith, 1993
  • Basic Statistics: An Introduction, George Summers, William Peters, Charles Armstring, 1977, p 218


About the Author

Mary Anne Thygesen is a freelance writer with over 40 years of experience. She regularly contributes to Taste of Home and Healthy Cooking magazines. She published in an education journal Microsoft Works in Education. She teaches cooking, nutrition, sewing, and quilting. She writes for eHow. She has a Bachelor of Science in mathematics from Portland State University.