Question 5.5: Use the data in Example 5.4 to find an 80% confidence interv...

To find an 80% confidence interval, set 1- α = 0.80 to obtain α = 0.20. The critical value is z._{10}, the z-score that cuts off 10% of the area in the right-hand tail. From the z table, we find that this value is z._{10} = 1.28. In Example 5.4, we found the point estimate to be \bar{X} = 12.68 and the standard error to be \sigma_{\bar{X}} =0.9659. The 80% confidence interval is therefore 12.68 ± (1.28)(0.9659) . This can be written as 12.68 ± 1.24, or as ( 11 .44, 13.92).