"Elmaache, Hamani" <Hamani.Elmaache@ccra-adrc.gc.ca>,
against my express wishes, wrote to me personally instead of to the
> I think that this problem is equvalent to:
> Estimate the sample size n necessary ( for Dichotomic Outcomes: 0 or
1) in order
> to test if the proportion (=P) is equal to p=0.02 given:
> 1) power(=1-BETA=0.8)
> 2) the specified ALPHA (=0.05)
> 3)EFFECT SIZE (0.005)
I am beginning to suspect that you are having this problem because
you are confusing the *difference*in*proportion* you wish
to detect with the *effect*size* . These are not at all the same.
Detecting a difference of 0.005 is a rational goal. Detecting an
effect size of 0.005 is *not*. Cohen considers something 40 times
larger to be a 'small' effect size.
> I know and I have the article of Cohen, but I don't know how does he
> calcul (or estimate the n).
> I tried to use SAS to estimate n, but SAS ask me to give it the
> interactive windows of SAS: Solutions/Analysis/Analyst) without
That's because SAS Analyst tool does not have a sample-size calculator
for a test of proportions. And Cohen focuses on tests on continuous
variables. In particular, for normally-distributed data, the sample
mean is statistically independent of the sample variance. For binomial
data, the sample proportion is *used* to compute the sample variance.
So defining the effect size is not the same problem.
Furthermore, you have only said that you wish to sample from 'a
How is this sample going to be achieved? Is this longitudinal data, or
a target population from which you will design a sampling regimen? Will
this be a simple random sample, or will there be sampling design
Is the population finite, or conceptually infinite? Your choices on the
sampling design will determine whether the classical sample-size
are even relevant. If you end up using, for example, a stratified
then the sample sizes needed for the individual strata may not be the
same, and the total sample size may be very different than that for a
simple random sample [in fact, it may be noticeably smaller if your
is designed well].
Perhap you could write to the list and give us some more details, so
that we might try to give you some more guidance. I'm not sure that
you are as yet asking the right questions.
Hoping I didn't sound too abrupt,
David Cassell, CSC
Senior computing specialist