Bioequivalence and Bioavailability Forum

Hi Angus!

❝ It seems that the English language description of the output you get is poor. Both WinNonlin and NCSS are from here.

You are absolutely right. Sometimes manuals increase confusion. PHC/WNL states in one place “variability” meaning standard deviation in one setting and variance in an other.

❝ The lack of clarity is the major problem and of course statistics is a confusing subject especially when the terms used are not well described or ccan be equivalent.

You are not alone.

❝ I compared the output from WinNonlin and NCSS: the variance term is exactly the same. WinNonlin calls the variance term "variance residual" NCSS provides a row in a Table and calls it the Error term of the 2 treatments A and B and it refers to it as the Mean Square Error (MSE) in another part of the output.

The latter is the most commonly one given by other software.

❝ They are in my example exactly the same number.

OK, fine. I was concerned about the way WNL calculates the confidence interval – which assumes equal variances. Only the degrees of freedom of the t-test have to corrected by the Satterthwaite approximation. If you want to estimate the sample size for another study you need just the residual variance, aka mean square error.

❝ So it seems that indeed it is the total error or total variance not the inter subject variance we have.

Yep.

❝ So that means that the value we have is a convolution of the intersubect error and intrasubject error?

I would rather call it a combination. Convolution smells of ∫…

❝ And as previously referred to is higher than intersubject variance so using it provides a higher estimate of sample size.

Almost. You could separate inter- and and intrasubject variance only in a cross-over study (see here). If you have data of a parallel study and want to plan another one simply use the error term. Call it CV_total if you are a nitpicker like me.