Question

The reserve capacities (in hours) of 18 randomly selected automotive batteries are listed. Use a 99% level of confidence. (Adapted from Consumer Reports) 1.70 1.60 1.94 1.58 1.74 1.60 1.86 1.72 1.38 1.46 1.64 1.49 1.55 1.70 1.75 0.88 1.77 2.07

Answer 1

99% confidence interval is (1.75, 5.61)

Explanation:

From the data above, mean is 3.68 and sd is 2.06. n = 18, degree of freedom = n-1 = 18-1 = 17. t-value corresponding to 17 degrees of freedom and 99% confidence level is 3.965

Confidence Interval (CI) = mean + or - (t×sd)/√n

CI = 3.68 + (3.965×2.06)/√18 = 3.68 + 8.1679/4.243 = 3.68 + 1.93 = 5.61

CI = 3.68 - (3.965×2.06)/√18 = 3.68 - 8.1678/4.243 = 3.68 - 1.93 = 1.75

99% confidence interval is (1.75, 5.61)