Confidence Level and Margin of Error. Example 16.1 (page 368) described NHANES survey data on the body mass index (BMI) of 936 young men. The mean BMI in the sample was x = 27.2. We treated these data as an SRS from a Normally distributed population with standard deviation o = 11.6. (a) Give three confidence intervals for the mean BMI / in this population, using 90%, 95%, and 99% confidence. (b) What are the margins of error for 90%, 95%, and 99% confidence? How does increasing the confidence level change the margin of error of a con- fidence interval when the sample size and population standard deviation remain the same