The best article I’ve read
about how to calculate power and sample sizes is Russell V. Lenth’s
“Some Practical Guidelines for Effective Sample Size Determination” in The
American Statistician (full reference below). It is
written for statistical consultants who assist researchers who need to
make sample size estimates, so it is just a bit on the technical
side. Since it is so well written, however, I recommend that
clients read it as well. Whether you are working with a
statistician who will do the calculations for you, or you are doing
them yourself, the article details the information you need to gather
and what you need to understand about the calculations.
Researchers who are at a
university that subscribes to JSTOR can read the article for free here
(for all others it is $14): http://www.jstor.org/pss/2685797.
Dr. Lenth's Power and
Sample-size web page has an applet that calculates power and sample
size, more information about calculating power and sample sizes, and a
version of the paper at:
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