9 Sample Size and Margin of Error. Example 16.1 (page 377) described NHANES survey the body mass index (BMI) of 654 young women. The mean BMI in the sample was 26.8. We treated these data as an SRS data on = from a Normally distributed population with standard deviation = 7.5. (a) Suppose that we had an SRS of just 100 young women. What would be the margin of error for 95% confidence? (b) Find the margins of error for 95% confidence based on SRSS of 400 young women and 1600 young women. (c) Compare the three margins of error. How does increasing the sample size change the margin of error of a confidence interval when the confidence level and population standard deviation remain the same?