Confidence Interval
Confidence interval and margin of error for a sample mean.
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The formula
CI = mean ± z · (sd / √n)
A confidence interval expresses sampling uncertainty as a range. The "95%" describes the method, not the interval: repeat the sampling many times and about 95% of the intervals produced would contain the true population mean. Larger samples narrow the range, but only by the square root.
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Frequently asked questions
What does a 95% confidence interval mean?+
If you repeated the sampling many times, about 95% of the intervals built this way would contain the true population mean. It is a statement about the method, not any single interval.
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