Question

A consumer agency is interested in examining whether there is a difference in two common sealant products used to waterproof residential backyard decks. With cooperation of several builders in the area, they randomly assign 38 newly constructed decks to be treated with Very Clear deck sealant and another 37 newly constructed decks to be treated with Sure Seal deck sealant. After one year of being exposed to similar weather conditions, the decks are rated on a scale of 1 to 100. The mean rating for the decks treated with Very Clear is 89.2 with a standard deviation of 3.1. The mean rating for the decks treated with Sure Seal is 92.4 with a standard deviation of 3.8.

Required:
What represents the 90 percent confidence interval to estimate the difference (Very Clear minus Sure Seal) in mean ratings for the two deck sealants?

Answer 1

(89.2 - 92.4) ± 1.666*√(3.1²/38 + 3.8²/37)

Explanation:

Very clear deck :

n1 = 38

x1 = 89.2

s1 = 3.1

Sure seal

n2 = 37

x2 = 92.4

s2 = 3.8

Confidence interval for difference in sample means :

(X1 - X2) ± margin of error

(X1 - X2) ± Tcritical * sqrt(s1²/n1 + s2²/n2))

Using the degree of freedom calculator ; df = 70

Margin of Error : Tcritical * sqrt(s1²/n1 + s2²/n2))

Tcritical at 90% confidence interval, df = 70 = 1.688

Margin of Error = 1.666 * sqrt(3.1²/38 + 3.8²/37)

Confidence interval :

(89.2 - 92.4) ± 1.666*√(3.1²/38 + 3.8²/37)