Question

Periodically, a town water department tests the the drinking water of homeowners for contaminants such as lead. The lead levels in water specimens collected for a sample of 10 residents of the town had a mean of 3.2 ?mg/L and a standard deviation of 2.9 ?mg/L. Complete parts a through c.

a. Construct a 90?%confidence interval for the mean lead level in water specimens from the town.

Answer 1

a)

Degrees of freedom df = n-1 = 10 - 1 = 9

t critical value at 0.10 significance level with 9 df = 1.833

90% confidence interval for is

- t * S * sqrt( n) < < + t * S * sqrt( n)

3.2 - 1.833 * 2.9 / sqrt(10) < < 3.2 + 1.833 * 2.9 / sqrt(10)

1.519 < < 4.881

90% confidence interval is (1.519 , 4.881)