Currently I'm doing a power calculation using the pwr package. I thought it should be very simple, but I'm not sure if I'm doing it right. I use R to calculate the sample size. I checked my results using other software. The results are really different. The documentation of the other software isn't available, that's why I post this question.
In short. I'm going to perform a two-sample, two-sided t-test. The no. of observations between the two groups are the same. I would like to measure/calculate a difference between the means of 4. The variance of my test is 12. The variance is equal between the groups. sig. level = 0.05 and the power is 0.8.
Using the pwr package I get the following code
d <- 4/12 pwr.t.test(n=NULL, d=d, sig.level=0.05, power=0.8, type="two.sample", alternative="two.sided")
With the following results
Two-sample t test power calculation
n = 142.2462 d = 0.3333333 sig.level = 0.05 power = 0.8 alternative = two.sided
NOTE: n is number in each group
However, the other software I use says that I just need 13 replications. Is the other package using another theory about calculating the no. of replications for a t-test? Or am I doing something wrong?
I hope you can help me out.
Best regards, Robin