Stats
Sample size for a binomial confidence interval (October 3, 2005).
Category: Sample size justification
Someone asked me for some help with a homework question. I hesitate to offer too much
advice in these situations because I don't want to disrupt the teacher's efforts to get the
students to think on their own.
If it is not too much trouble, I would really appreciate your kind assistance on
how best to determine and calculate for sample size if the only information given to me
is the estimate (i.e. 10% of all male patient in the study population likely to be
screened for prostate cancer at the medical clinic) and the precision of this estimate
(i.e. 5%).
This sure sounds like they want a sample size that will produce a 95% confidence interval
that has a width of plus or minus 5%. The confidence interval is based on a single binomial
distribution, I suspect, rather than a difference in two binomial proportions. There's a bit
of ambiguity in the wording, so it always helps to get a bit of clarification.
I have a spreadsheet that does confidence interval calculations for a single binomial
proportion and you can play some "what-if" games to arrive at an appropriate sample size. But
there are also formulas that you can use. Here are two web sites that provide simple and easy
to follow guidance.
07/08/2008.
