At 13:40 19/01/99 -0000, Ted Harding wrote:
>Nico van Duijn is being provocative ... !!
>
And providing some good thoughts: I am enjoying this thread very much!
snip
>
>Alpha, beta and the differencefor which beta woud be obtained are in
>principle pre-trial design factors, which predict the performance of the
>trial as designed (in practice you may not have a good notion of them
>numerically until you have had the chance to analyse the data, but that
>is another story).
>
>However -- and here I find myself, I think, beside Nico -- one meets
>cases like the following. There is a limited number of patients available:
>say "up to 60" or "up to 200".
snip
>the two treatments, there will be a beta for each alpha. You can turn
>this round and calculate the difference X such that the trial, using
>critical significance level alpha=0.05 (say) has power beta=0.80 (say).
>Trial reports tend to be unacceptable for publication unless they include
>a "power calculation". So the protocol includes a section which concludes
>"At 5% significance, the trial has 80% power to detect a difference of X"
>(but see below).
In the ideal world one would like to go to enough numbers to establish high
alpha *and* beta, and where this is possible this is the preferable
approach. Where numbers are limited (porbaly the default option)the advice
often tends to be "dont bother doing the trial your beta is too low" (for
conventional alpha); this is where I think Nicos ideas have merit.
[log in to unmask] <Brian Stein>
Human Immunology, Hanson Centre for Cancer Research
IMVS, Frome Road
Adelaide SA 5000 voice: +61 8 8222 3725
Australia fax: +61 8 8232 4092
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