Basic & quick question about confidence interval (novice) I'm writing a research proposal and I need to include an estimation of what kind of sample size would be adequate.
I've used a calculator, and I get the result that with a 95% confidence interval and 5% confidence level, the sample size should be at least 384. I don't know my population but I know that it's very large, and I've read that if that's the case, the population size is irrelevant and the answer will always be 384.
Is this correct, and do I need to explain this logic in a research proposal? Are a 95% confidence interval and a 5% confidence level adequate for most studies? (clinical trial in question).
I know the questions are very basic - I'm a complete beginner. Would be so grateful for any help.
 A: Caveat: I am answering the question I think you're asking. It's somewhat unclear, so this may be inaccurate.
The calculator you are using is asking for the confidence interval for a proportion - for example, "87% of Respondents are X +/- 3%". That "3" is what goes in the confidence interval section. It appears you put in "5" there, and yes, for arbitrarily large population that number will continue to be 384 (you can prove this to yourself by putting different population values into the calculator).
However, I have a couple concerns:


*

*Do you know your population is actually "very large"? What is your population? "Very large" isn't well defined.

*You haven't talked about your actual study at all. We cannot answer for you if that's correct for "most studies" because it will very much depend on what you're actually asking. You should have most of your analysis plan already in place before you start trying to calculate your needed sample size.

*Sample size calculations are often somewhat tricky - do you have an advisor or someone else who could talk you through this?

