A doctor wants to estimate the mean HDL cholesterol of all20- to29-year-old females. How many subjects are needed to estimate the mean HDL cholesterol within 3

3 points with 99 %

99% confidence assuming s equals 15.4

s=15.4 based on earlierstudies? Suppose the doctor would be content with 95 %

95% confidence. How does the decrease in confidence affect the sample sizerequired?

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Level ofConfidence, (1minus

alpha

)times

100%

Area in EachTail, StartFraction alpha Over 2 EndFraction

2

CriticalValue, z Subscript alpha divided by 2

Level of confidence area in each trial critical value

90% .05 1.645

95 % .025 1.96

99% .005 2.575