solution

For the data in the previous exercise, test the hypothesis that the 0.8 quantile regression line has a zero slope with leverage points removed.

The author’s web page contains the file skull data.txt, which contains data reported by Thompson and Randall-Maciver (1905) dealing with skull measurements of male Egyptians from different time periods. The four measurements are contained in the first four columns and the time period is indicated by the values in column five. Using the default values for the arguments, create a smooth using lplot and the data in columns one (the independent variable) and columns 4. What does the smooth suggest regarding curvature? Check this issue using linchk.