CV-intra # 60% CV-inter [Power / Sample Size]

posted by Helmut Homepage – Vienna, Austria, 2007-07-22 03:17 (5980 d 06:55 ago) – Posting: # 921
Views: 22,854

Dear Nav,

I’m not in my office, so be a little patient to get a thorough response – first I have to read your 66 pages of statistics. ;-)

Just for completeness (because you sent me the paper by private mail):

Julious SA. TUTORIAL IN BIOSTATISTICS. Sample sizes for clinical trials with Normal data.
Stat Med. 2004;23(12):1921–86. doi:10.1002/sim.1783.

❝ Well, I am not sure how to estimate the sample size for a parallel design?

We’ll handle that… ;-)

❝ Re: 60% of inter to estimate the intra: I am a statistician and I routinely see the PK individuals taking 60% of inter to be the intra for estimating sample size for a crossover design and always wondered about the validity of the approach.

Hey, that’s black magick!
  1. Imagine two studies; one with very tight inclusion criteria with respect to anthrompometric covariates (sex, BMI, age, …), the other one very ‘liberal’. CVinter (but not CVintra!) will be higher in the first one; just my 2 ¢.
  2. If I remember it correctly, diltiazem is a compound with low to moderate intra-subject CV (about 15%), but much higher CVinter.
  3. Extreme examples are all studies employing different phenotypes. Whereas the phenotype is time-invariant – we don’t change our genotype in the washout – the intra-subject variability may be horrible (20fold differences in Cmax are not uncommon).
Lessons from the examples:
  1. Although you may have established some kind of intra-/inter-CV ratio, this most likely is not – only – a property of the compound / formulation(s), but also of the study’s design/performance.
  2. No, not 60%, rather 25%…
  3. Again from my aging memory: buspirone CVintra ~15%, CVinter >80%?
Therefore I would consider

❝ 60% of inter to estimate the intra

again as black magick. :wink:

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