Bioequivalence and Bioavailability Forum

Hi Vipraj,

❝ I was thinking that S2w (S square w) is within subject variance and Sw is within subject variability (assumption was based on the formula mentioned for variance and standard deviation in various literature) . Pl. confirm if it is true or false.

Yes. In fact, the step 1 of method description before the SAS code in FDA's guidance already clearly mentioned it as "Determine S_WR, the within-subject standard deviation (SD) of the reference product, for the pharmacokinetic (PK) parameters AUC and Cmax. ..."

❝ Also i was interested in knowing how we can calculate S2w (S square w) from available Sw value.

Now I'm lost. You mean other way than just square it?

Back to your original question

❝ Is there any difference between within-subject variance and within-subject variability ?

Like Helmut said, depending on the context. Sometimes they mean the same thing, sometimes they don't.

In the "additional comments" of the same FDA guidance, they tell us in order to apply the scaled method the criterion is "within-subject variability ≥ 30%)", so clearly here the variability is not equal to variance since you need to do sqrt(exp(s2w) - 1) to get it.