Power analysis for patient samples

Question

[mropri]

Hi,

I am trying to calculate power for 150 samples where 75 are going to be in one group and 75 in another. I tried using the pwr package in R to get the power where I used the following code:

pwr.anova.test(k=2, n =75, f=.1, sig.level=.05, power=NULL)

k for me equals 2 because the 150 samples are divided into two groups equally (75 each)

n which is the sample size is 75 for both groups

f is the effect size which I set to 0.1

significance level I set to 0.05

I get a number for power which is 0.23 which obviously is low. My question is, am I doing this correctly? I am new to power calculations and if someone could help me understand how to calculate power knowing the sample size or point me to a package that can correctly do this for my example, I would truly appreciate this.

[Jeremy]

I think you're doing it right. Since you only have 2 groups, you can use pwr.t.test. The default alternative hypothesis is two-sided, but if you are only looking for an increase or decrease, then you can use a one-sided alternative hypothesis. Alternatively, you can use power.t.test, but you will need to proved an estimated standard deviation.

[mropri]

Sounds good, thanks Jeremy for the help, really appreciate it.