# Sample size from Standard deviation and population proportion

Obtained a sample size from standard deviation and another sample size from proportion, which one should I use if I need to be mindful not to propose a more costly study than necessary?

$n = 74$ (from standard deviation) and $n = 358$ (from proportion)

such that $37\%$ is the population proportion the standard deviation of annual income in this population is around $22000$. $95\%$ confidence interval $\pm0.05$ for the proportion with a post-secondary credential $95\%$ confidence interval with width no more than $\$5,000$for the average income in the population. • Your question is unclear. Are you saying that you obtained estimates of standard deviation and mean from two DIFFERENT samples? (which is unusual), and you want to know what sample size to use to get a confidence interval? If you gathered two sets of samples, you could always combine them and get an overall estimate of standard deviation which is what you need to determine how many samples to use for a prescribed confidence interval (assuming distribution is Gaussian). – user2566092 Sep 19 '15 at 19:18 • There are numbers in the problem that might lead to two different kinds of analysis. As a result, I think you have gotten confused trying to use two different formulas at once for the for sample size$n$. Please read my Answer. If it helps clarify the issues, fine. If not, try to clarify your question and maybe someone can help. – BruceET Sep 19 '15 at 21:01 • Goal: To produce estimates of both average annual income and proportion with PSC in a target population. A simple random sample from the population will be used. Informed pre-study guesses are that (i) 37% of this population has a PSC, and (ii) the standard deviation of annual income in this population is around$ 22000. – Tree Sep 19 '15 at 22:14