19.1: Power Analysis

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.