Expertise:
Beginner
What do we need to think about next?
Note that the curves seem to fit the height observations exactly. This process
was simple, but is fitting the data exactly a good idea? We already acknowledged
in the "Introduction to the Data" web page that the error in height
measurements is of the order of three millimeters, so shouldn't we try to find
curves that get closer to the truth rather than fitting the noisy data exactly?
To see how to do this, go to the next sample analysis of these data where we
consider smoothing.
However, a simpler method to control the smoothness of a fit is to limit
the number of basis functions. If knots are not specified, but the number of
basis functions are, then knots are placed at equally spaced points. We know
that about twelve basis functions are required to give a reasonable account of a
growth curve. We could modify the basis creation command above to
hgtbasis = create_bspline_basis([1,18], 12, 6);
