# Sample size calculation when comparing two coefficients of variation

#### sirgeensnor

##### New Member
Hi,

I am a medical student, wanting to start a clinical study between two devices. Since I want to compare especially on precision and reproducibility, I am going to use the coefficient of variation. I already have the CV of one of the devices from another published article (14,6%) and have calculated the other device will give me a CV of 11%.
Which method do I use to calculate my sample size that is needed to power this study? At this moment I use G*Power for my calculations.

Thank you and have a great weekend.

#### fed2

##### Active Member
is the measurement continuous, count, normal, lognormal or what? Need to have some idea of that. I'd suggest that this will work out best under log-normal model.

#### katxt

##### Active Member
Assuming you want to show that the second is more precise than the first, you can use a Monte Carlo approach. Choose a possible sample size n. Generate n normal(100, 11). Calculate CV for your sample. Record 1 if the sample CV is less than 14.6% else 0. Repeat many times. The average of the 0, 1s is the power. If it is too low, increase the sample size and try again. This is a one tailed test as it stands.

#### katxt

##### Active Member
This little known formula may also help. For reasonable values of CV and n, SE(CV)=CV/sqrt(2n)

#### fed2

##### Active Member
im pretty sure the OP just wanted to know what button to push on G*Power. Poor bugger....