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st: Re: statalist-digest V4 #1393

From   Paul Seed <[email protected]>
To   [email protected]
Subject   st: Re: statalist-digest V4 #1393
Date   Wed, 08 Oct 2003 18:24:03 +0100

>Jannik Helweg-Larsen wrote:
>Dear list
>Using Stata 8.1
>I'm unsure on how to calculate the required sample for the following trial,
>been searching by findit for equivalence ado's without finding my answer:
>We are planning to do a 2 year equivalence trial.The current standard of
>treatment has a 2 year failure rate of 7%: In our trial of a new drug, the
>treatments will be considered equivalent if the 95% confidence interval are
>within the bound of -12% to 12%, which we believe is the  the largest
>difference that would be considered clinically acceptable.
>any suggestions?
As has been pointed out, an absolute difference in risk of +/- 12% would swamp the
treatment effect. I suspect that what is meant is a relative risk reduction of +/-12%.

Using this, 7% would be reduced by the new treatment to 0.07*0.88 = 6.16% or increased to
7.84%. Only the second figure is of interest.

We can now use sampsi, assuming we are interested in one-tailed tests relating to an
increase only, and will accept 80% power, as follows:

sampsi .07 .0784, onesided power(.8)

The answer given is very large: over 12,000/group, nearly 25,000 in all.
However, such trials have been organised.

One alternative would be to use a surrogate endpoint, rather than failure (Y/N).

Paul T Seed ([email protected])
VIP Trial Statistician/Data Manager

Department of Obstetrics & Gynaecology
Guy's Kings and St. Thomas' School of Medicine, King's College London,
St Thomas' Hospital,
Lambeth Palace Road,
London SE1 7EH

tel (+44) (0) 20 7960 5889
fax (+44) (0) 20 7620 1227

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