>>> Ted Harding <[log in to unmask]> 03/27/06 3:08 AM >>>
> Also, does it follow that if a trial is repeated X times,
> the observed point estimate for each trial be normally distributed
> within the confidence interval of an adequately powered trial?
No. First of all, an "adequately powered" trial may be so much
adequately powered that the CI for that trial is very short indeed.
However, the distribution of the point estimate for a trial
differently powered may be scattered ofver a much wider range.
Secondly, suppose the point-estimate methodology is unbiased.
Now take the CI for the "adequately powered trial". The
"midpoint" will be the point estimate from that trial. Even
if the methodology is unbiased, that particular value will
have an error. So the particular CI you have obtained from
that trial will be offset relatively to the true value of
the parameter. Hence, when you embark on the X repetitions,
you will obtain a series of estimates which, overall, are
unbiasedly distributed relative to the true value. Hence
their distribution will be offset relative to the CI you
obtained from the "adequately powered trial".
HM : I think this principle is well shown in a graphical representation
of the results of simulated trials in this essay :
Scroll down until you see the graphical representation of the C.I.s of
100 simulated trials (about two-thirds of the way down the page).
I'd appreciate any thoughts about the general assertions concerning the
problem of low event rates in RCTs, discussed in great detail in the
(Disclosure: The author is my brother-physician.)
University of Utah