In a study on infants, one of the characteristics measured was head circumference. The mean head circumference of 15 infants was 34.8 centimeters (cm). Complete parts

(a) through

(d) below.

a. Assuming that head circumferences for infants are normally distributed with standard deviation 2.1 cm, determine a 90% confidence interval for the mean head circumference of all infants.

b. Obtain the margin of error, E, for the confidence interval you found in part (a).

c. Explain the meaning of E in this context in terms of the accuracy of the estimate. Choose the correct answer below and fill in the answer box to complete your choice.

d. Determine the sample size required to have a margin of error of 0.9 cm with a 95% confidence level.