Page 233 - Six Sigma Demystified

Part 3 s i x s i g m a to o l s 213

Confidence Interval on Proportion

Minitab

Menu: Stat\Basic Stats\1 Proportion
“Options” button: Set Confidence Level and alternative Not Equal (two-sided
case) or > or < (for one-sided cases); by default uses exact method (not normal
approximation).

Two-Sided Example at 95% Confidence

Test and Ci for One Proportion
Sample X N Sample p 95% CI
1 24 100 0.240000 (0.160225, 0.335735)

One-Sided Example at 95% Confidence (Using Alternative

greater than)

Test and Ci for One Proportion
95% Lower
Sample X N Sample p Bound
1 24 100 0.240000 0.171348

Excel

Using the normal distribution assumption, calculate in cell using p , s, and n
with NORMSINV function, where

p −(1 p)
s =
n

Upper = p + NORMSINV(1 – α/2) × s

Lower = p + NORMSINV(α/2) × s

(Continued)