4. Refer to Example 51 and the responses about the number of male sex partners. Of the 1431 subjects responding with a positve number, the mean was 4.8 and the standard de- viation was 6.2.

a) Verify that the 95% confidence interval for that population mean equals (4.5.5.1).

b) What statistical factors cause the interval in

(a) to be narrower than the 95% confidence interval reported in Example 5 1 for tlie mean number of female panners?