Confidence interval

Compute a t-based confidence interval for a mean from sample stats at 90/95/99%.

CI = x̄ ± t(s/√n) using the t distribution with n−1 degrees of freedom — appropriate for a sample SD. The interval describes the population mean, not individual values.

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How to use

  1. Enter the sample mean, sample SD, and sample size.
  2. Pick 90%, 95%, or 99%.
  3. Read the interval, margin of error, and standard error.

What it supports

CI = x̄ ± critical × (s/√n): t distribution at n−1 df for a sample SD (n ≥ 3), or z for a known σ (n ≥ 2). SD ≥ 0. The interval covers the population mean, not individual observations; proportion intervals are not included.

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