## No relationship between CVinter and CVintra [Power / Sample Size]

Hi Sridhar,

» can any one help how i convert inter subject CV to intra subject CV for sample size estimation.

What do you mean by ‘convert’? It’s like saying ‘convert an apple to an orange’. Both are approximately spherical fruit, more or less sweet and of similar weight. However, they are exclusively distinct.

If you have a crossover study, both CVs are accessible. If you have a parallel design, you get only the total (aka pooled) CV (see this article). There is an infinite number of combinations of CVinter and CVintra which will give the same CVtotal.
$$\begin{matrix} s_\textrm{inter}^2=\log_{e}(CV_\textrm{inter}^2+1)\\ s_\textrm{intra}^2=\log_{e}(CV_\textrm{intra}^2+1)\\ CV_\textrm{total}=\sqrt{\exp\left(s_\textrm{inter}^2+s_\textrm{intra}^2-1\right)} \end{matrix}$$

“We added two numbers and their sum was five. What were the two numbers?”

Leaves the pupil – rightly – dazed and confused.

Dif-tor heh smusma 🖖
Helmut Schütz

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