An investigator computes a 95% confidence interval for a population mean on the basis of a sample of size 70. If she wishes to compute a 95% confidence interval that is half as wide, how large a sample does she need?
What will be an ideal response?
With a sample size of 70, the standard deviation of 
. To make the interval half as wide, the standard deviation of 
will have to be 
. The sample size needs to be 280.
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