The National Health and Nutrition Examination Survey (NHANES) collects demographic, socioeconomic, dietary, and health related information on an annual basis. Here is a sample of 20 observations on HDL cholesterol level (mg/dl) obtained from the 2009-2010 survey (HDL is "good" cholesterol; the higher its value, the lower the risk for heart disease):

a. Calculate a point estimate of the population mean HDL cholesterol level.
b. Making no assumptions about the shape of the population distribution, calculate a point estimate of the value that separates the largest 50% of HDL levels from the smallest 50%.
c. Calculate a point estimate of the population standard deviation.
d. An HDL level of at least 60 is considered desirable as it corresponds to a significantly lower risk of heart disease. Making no assumptions about the shape of the population distribution, estimate the proportion p of the population having an HDL level of at least 60.

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