Formula for calculating the power of a test

[Ekoue]

Hi all,

There is a simple formula for calculating the sample size. Is there a formula for calculating the power of a test?

Regards.

Ekoue

[Thompson,Paul]

There are many many. You need to be specific.

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[Swank, Paul R]

If you have the appropriate formula for the sample size given the power, simply plug in the sample size and solve for the power.

Sent from my iPad

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[BXC (Bendix Carstensen)]

However, normally it would be much more informative to know the *precision* of your estimates, as for example the width of the confidence interval of the estimate of the parameter you are estimating.

For that purpose the best approach is to simulate a dataset that looks like the one you envisage to collect, analyse it as you envisage to analyse your data, and then look at the width of the c.i. You repeat this say 1000 times, and then look at the median width of the c.i. Incidentally, the fraction of significant tests among the 1000 simulations is the power.

This procedure ensures that you actually have specified all your assumptions, because you will not be able to simulate your envisaged dataset otherwise.
Some consider this major drawback too.

b.r.
Bendix Carstensen

[Barry W Brown]

We post two free programs for relating sample size to power or length of confidence interval for many

(twenty odd for the power calculations) testing situations.  Power is via STPLAN, the cother is CONFINT.

biostatistics.mdanderson.org/SoftwareDownload.