Ch7 9(d) bs(4) has *ONE* knot

[SONGDONGYUAN1994]

1. First, bs(4) should have only ONE knot;
2. Second, it seems like when adding both df and knot in bs(), the result is not just ignoring knot.

[leowang396]

agreed.

also, plot of cubic spline fit with 4 degrees of freedom behaves quite well at extreme dis values, not showing the wiggly tails in seen in the proposed answer.