estimate-with-margin-of-error

Estimate a true average or rate from a sample and say how precise it is, or work out how big a sample is needed. Always use this skill when someone asks for a confidence interval or margin of error, what a survey, audit, inspection or quality sample says about the whole population, how many records, customers or units to sample for a target precision, whether a sample is big enough to trust, whether to use a t or z interval for a small sample, or how to explain a confidence interval to a manager, even when the question sounds like a quick check. Also use it for intervals for proportions with small counts or rates near zero or one, finite populations, and sampling pitfalls such as convenience samples and non-response. Not for comparing two groups, not for sizing an A/B test, and not for explaining what a confidence interval means when no data is in play.