title: how do I select the smoothing parameter for smooth.spline()?
body: <p>I know that the smoothing parameter(lambda) is quite important for fitting a smoothing spline, but I did not see any post here regarding how to select a reasonable lambda (spar=?), I was told that spar normally ranges from 0 to 1. Could anyone share your experience when use smooth.spline()? Thanks.</p>
  
  <pre><code>    smooth.spline(x, y = NULL, w = NULL, df, spar = NULL,
            cv = FALSE, all.knots = FALSE, nknots = NULL,
            keep.data = TRUE, df.offset = 0, penalty = 1,
            control.spar = list(), tol = 1e-6 * IQR(x))
  </code></pre>