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Statistics LibreTexts

19.1: Power Analysis

  • Page ID
    8814
  • We can compute a power analysis using functions from the pwr package. Let’s focus on the power for a t-test in order to determine a difference in the mean between two groups. Let’s say that we think than an effect size of Cohen’s d=0.5 is realistic for the study in question (based on previous research) and would be of scientific interest. We wish to have 80% power to find the effect if it exists. We can compute the sample size needed for adequate power using the pwr.t.test() function:

    pwr.t.test(d=0.5, power=.8)
    ## 
    ##      Two-sample t test power calculation 
    ## 
    ##               n = 64
    ##               d = 0.5
    ##       sig.level = 0.05
    ##           power = 0.8
    ##     alternative = two.sided
    ## 
    ## NOTE: n is number in *each* group

    Thus, about 64 participants would be needed in each group in order to test the hypothesis with adequate power.