Question

Fo quality control purposes, we collect a sample of 300 items and find 36 defective items in it. Construct a 90% confidence interval [a, b] for the proportion of defective items in the whole shipment.

Answer 1

(0.089 ; 0.151)

Explanation:

Given :

Sample size, n = 300

Number of defective items, x = 36

The confidence interval required here is that for a one sample proportion :

The confidence interval is defined thus :

Phat ± Zcritical * √[Phat(1 - phat) / n]

Zcritical at 90% = 1.645

Phat = x / n = 36 / 300 = 0.12

Hence,

C.I = 0.12 ± 1.645 * √[0.12(1 - 0.12) / 300]

C.I = 0.12 ± (1.645 * 0.0187616)

C.I = 0.12 ± 0.0308629

C.I = (0.089 ; 0.151)