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Dear all,
I am new to this clinical field. Any one help me to clear some doubts
in sample size calculation for a 2 x 2 crossover design.
I know only the Cmax value of the drug from the literature. It is
around 42.34plus or minus 13.27.
How I can calculate the intra subject CV from this Cmax value? (is it
required for Sample Size calculation?)
which value of "delta" we commonly used for calculating Sample Size?
there are different sample size formulas for addictive model and
multiplicative model? which model we commonly used? why?
Mathews
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Dear Mathew,
You need an estimate of the intra-subject CV from ANOVA for
calculating the sample size. However, you can get a ball park
estimate of the intra-subject CV by taking 60% of the inter-subject
variability (sd/mean). I personally do not use this method because
its not robust.
Please see the paper by Steven A Julious, "Tutorial in Biostatistics
- Sample sizes for clinical trials with normal data." Its an
excellent reference.
Nav Coelho
Biovail Contract Research
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The following message was posted to: PharmPK
Dear Navdeep & Mathews
> ... However, you can get a ball park
> estimate of the intra-subject CV by taking 60% of the inter-subject
> variability (sd/mean). I personally do not use this method because
> its not robust.
I don't think this '60%-rule by-thumb' is of any good. ;-)
See this two posts:
http://forum.bebac.at/forum_entry.php?id=921
http://forum.bebac.at/forum_entry.php?id=925
Best regards,
Helmut
--
Ing. Helmut SchA\0x00tz
BEBAC - Consultancy Services for
Bioequivalence and Bioavailability Studies
Neubaugasse 36/11
1070 Vienna, Austria
e-mail helmut.schuetz.aaa.bebac.at
web http://bebac.at
forum http://forum.bebac.at
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The following message was posted to: PharmPK
Hi Mathews,
I developed a free tool in Excel to calculate sample sizes a little
while
ago. It's available here:
http://individual.utoronto.ca/ddubins/FARTSSIE.zip
I don't know where the 60% CV idea came from. Maybe it came from the
fact
that the total variance is the sum of the between- and within-subject
variances:
Sigma^2 = sigmab^2 + sigmaw^2
As a worst-case scenario, if the intra-subject variance is assumed to be
equal to the inter-subject variance, then the equation reduces to:
Sigmaw = sqrt(sigma^2 / 2) = sigma / 1.212 = 70.7% sigma
I've seen some people use 60% total CV, and others use 50%. In the
end, you
have to work with the data you have. If all else fails - run a pilot
study
to find the true ratios and variability of the test formulation. This
provides the best sample size estimate.
Hope this helps,
Dave Dubins, B.A.Sc., Ph.D.
Director Biopharmaceutics - Bio Pharma Services Inc.
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