Question

From her purchased bags, Rory counted 110 red candies out of 550 total candies. Using a 90% confidence interval for the population proportion, what are the lower and upper limit of the interval? Answer choices are rounded to the thousandths place.

Answer 1

Lower Limit: 0.172

Upper Limit: 0.228

Explanation:

Answer 2

The Confidence Interval = (0.172, 0.228)

Where:

The lower limit = 0.172

The upper limit = 0.228

Explanation:

The formula to be applied or used to solve this question is :

Confidence Interval formula for proportion.

The formula is given as :

p ± z × √[p(1 - p)/n]

n = Total number of red candies = 550 red candles

p = proportion = Number of red candies counted/ Total number of red candies

= 110/550 = 1/5 = 0.2

z = z score for the given confidence interval.

We are given a confidence interval of 90%. Therefore, the z score = 1.6449

Confidence Interval = p ± z × √[p(1 - p)/n]

Confidence Interval = 0.2 ± 1.6449 × √[0.2(1 - 0.2)/550]

= 0.2 ± 1.6449 √0.2 × 0.8/550

= 0.2 ± 1.6449 × 0.0170560573

= 0.2 ± 0.0280555087

Hence, the Confidence Interval = 0.2 ± 0.0280555087

0.2 - 0.0280555087 = 0.1719444913

Approximately = 0.172

0.2 + 0.0280555087 = 0.2280555087

Approximately = 0.228

Therefore, the Confidence Interval = (0.172, 0.228)

Where:

The lower limit = 0.172

The upper limit = 0.228