Question

The EPA limit on the allowable discharge of suspended solids into rivers and streams is 60 milligrams per liter (mg/l) per day. A study of water samples selected from the discharge at a phosphate mine shows that over a long period, the mean daily discharge of suspended solids is 48 mg/l, but day-to-day discharge readings are variable. State inspectors measured the discharge rates of suspended solids for

n = 20

days and found

s² = 32 (mg/l)².

Find a 90% confidence interval for

σ².

(Round your answers to three decimal places.)

?? to ??

Answer 1

Solution :

Given that,

Point estimate = s² = 32

²_R = ²_/2,df = 30.144

²_L = ²_{1 - /2,df} = 10.117

The 90% confidence interval for ² is,

(n - 1)s² / ²_/2 < ² < (n - 1)s² / ²_{1 - /2}

19 * 32 / 30.144 < ² < 19 * 32 / 10.117

20.170 < ² < 60.097

20.170 to 60.097