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210 Six SigMa DemystifieD

Confidence Interval on Mean

Standard Deviation unknown

Minitab

Menu: Stat\Basic Stats\1 Sample t

“Options” button: Set Confidence Level and alternative Not Equal (two-sided
case) or > or < (for one-sided cases).

Two-Sided Example at 95% Confidence

One-Sample T
N Mean SD SE Mean 95% CI
25 25.700 1.800 0.360 (24.957, 26.443)

One-Sided Example at 95% Confidence
(Using Alternative greater than)

One-Sample T
95% Lower
N Mean SD SE Mean Bound
25 25.700 1.800 0.360 25.084
Excel

Calculate in cell using calculated values of X , s, and n with the TINV function.
(Note: Excel’s TINV function assumes a two-sided case and interprets entered
α as α/2).

Upper = X + TINV(α, n – 1) × s/SQRT(n)

Lower = X – TINV(α, n – 1) × s/SQRT(n)

Two-Sided Example at 95% Confidence

Upper (results in calculated value of 26.443) = 25.7 + TINV(0.05, 24) × 1.8/SQRT(25)

Lower (results in calculated value of 24.957) = 25.7 – TINV(0.05, 24) × 1.8/SQRT(25)