Use the data in file nhanes.dat for this exercise. (If you prefer, you may download the NHANES data from the website at www.cdc.gov/nchs/nhanes.htm.) Triceps skinfold measurements are sometimes used as a gauge of body fat. We are interested in the relation between y=triceps skinfold (variable bmxtri) and x =body mass index (variable bmxbmi).
a. Estimate the mean value of triceps skinfold for the population, along with a 95% CI.
b. Draw a histogram of the variable triceps skinfold, using the weights. Do the data appear to be normally distributed?
c. Find the minimum, 25th, 50th, and 75th percentiles, and maximum for the variable triceps skinfold. Calculate the same quantities separately for each gender (variable riagendr). Use these to construct side-by-side skeletal boxplots of the data as in Figure 7.7.
d. Construct a weighted circle plot with smoothed trend line for y variable triceps skinfold and x variable body mass index. Does there appear to be a linear relationship? What other features do you see in the data?