Not knowing the standard deviation of the population we can only use the sample value 40, even though we know it's wrong. In the above we go ahead as though we know the population standard deviation, even though we usually don't. Suppose 1000 hospital records show 480 female births and 420 male births. We don't know the standard deviation, but we can approximate with the sample standard deviation. Score girls as 1 and boys as 0 .
Exercises Repeat the above for boys instead of girls.
Remember that boy is coded as 1-girl. Calculate a 2 sigma interval using the data for 10,000 births mentoned above.