low-up for the patients randomized in the trial. The <code>id</code> variable identifies measurements taken on the same patient. We would like to investigate the placing of the knots in a natural cubic spline for the follow-up times for the log-odds of high serum cholesterol levels. In the first GEE we place two knots at the median and third quantile (the first quantile is equal to the boundary knot at zero). In the second GEE we place the knots at two, five and seven years of follow-up. The working correla