A doctor wants to estimate the HDL cholesterol of all 20-to 29- year-old females. How many sunjects are needed to estimate the HDL cholesterol within 3 point with 99% confidence assumming ?=18.6?
Suppose the doctor would be content with 90% confidence. How does the decrease in confidence affect the sample size required?
A 99% confidences level level requires ___subjects.
(Round up to the nearest whole number as needed.)