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208 Six SigMa DemystifieD

Confidence Interval on Mean

Standard Deviation Known

Minitab

Menu: Stat\Basic Stats\1 Sample z
“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 Z
The assumed standard deviation = 1.8.
N Mean SE Mean 95% CI
25 25.700 0.360 (24.994, 26.406)

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

One-Sample Z

The assumed standard deviation = 1.8.
95% Lower
N Mean SE Mean Bound
25 25.700 0.360 25.108

Excel

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

Upper = X + CONFIDENCE(α, s, n)

Lower = X – CONFIDENCE(α, s, n)