A doctor wants to estimate the mean HDL cholesterol of all 20-to 29 years-old females. How many subjects are needed to estimate the mean HDL cholesterol within 4 points with 99% confidence assuming s=15.9 based on earlier studies? Suppose the doctor would be content with 90% confidence. How does the decrease in confidence affect the sample size required?

Level of Confidence

(1-a)*100%

90%

95%

99%

Area in Each Tail a/ over 2

0.05

0.025

0.005

Critical Value za/ over 2

1.645

1.96

2.575