CVtotal ≠ CVintra + CVinter [Power / Sample Size]

posted by Helmut Homepage – Vienna, Austria, 2015-05-15 15:05 (3563 d 13:59 ago) – Posting: # 14827
Views: 12,328

Hi John John,

❝ In a bioavailability study with six healthy volunteers:


I would never plan a pivotal study based on results of such a small pilot – especially if the variability is high. Let’s assume a CV of 30%. From six subjects we can calculate a confidence interval of the CV with 18–102%. Not meaningful.

...(Cmax; range 135–422 ng ml-') and time to maximum (tmax; range 1.5–7.0 h) for the three tablet doses showed significant differences between subjects (P <0.009; between subject coefficients of variation (CVs) 34% and 27%, respectively). There were no within subject differences in C, (P = 0.32; mean within subject CV 11%


The quoted text leaves many questions open. I guess it was a crossover. Significant differences between subjects… Why report this at all? Since we know that results come from different sub­jects, this effect will  always  likely be significant.
[image]Two values for CVinter? How were those calculated? From the ANOVA you get only one. Some people use the (arithmetic!) SD/mean of treatments (like in the crappy paper above). Has nothing to do with the ANOVA on log-transfomed data.

❝ To calculate the sample size for a parallel study I use the total CV= 45?


No way! CVs are not additive (CVpooled  CVintra + CVinter). See the methylphenidate example from this post.

  MSe      MSs   CVintra CVinter CVpooled
0.004882 0.07675 7.00%   19.1%   20.4%


❝ or do I need the pooled variability?


Yes.

❝ Which is?


Not the slightest idea. Not accessible from what you posted. You need either the complete ANOVA or the raw data.

Dif-tor heh smusma 🖖🏼 Довге життя Україна! [image]
Helmut Schütz
[image]

The quality of responses received is directly proportional to the quality of the question asked. 🚮
Science Quotes

Complete thread:

UA Flag
Activity
 Admin contact
23,380 posts in 4,914 threads, 1,665 registered users;
68 visitors (0 registered, 68 guests [including 7 identified bots]).
Forum time: 04:04 CET (Europe/Vienna)

When people learn no tools of judgment
and merely follow their hopes,
the seeds of political manipulation are sown.    Stephen Jay Gould

The Bioequivalence and Bioavailability Forum is hosted by
BEBAC Ing. Helmut Schütz
HTML5