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Data & Analytics · Intermediate

Confidence Interval (CI)

A range computed from sample data that, under repeated sampling, would contain the true population parameter a stated percentage of the time.

Definition

A 95% CI for a mean is roughly x̄ ± 1.96 × SE (or t × SE for small samples). Wider intervals come from higher confidence, more variability or smaller samples.

Example

Average delivery time 3.2 days, 95% CI [3.0, 3.4].

Exam tip

A 95% CI does not mean a 95% probability that this particular interval contains the parameter; the 95% describes the procedure.

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