Thanks for your helpful suggestions Maarten,
Actually my main concern *is* with finding the location of the one
knot. I could provide a guess to start things off, but need to know the
best fit location.
I am not interested in any old flexible line, the model is following a
theoretical constraint - it has to be two straight lines. Where they
intersect is an important parameter which we will then (having
estimated it) feed into another model. As I understand it mkspline does
not handle such a problem, but your use of nl may do.
On Tuesday, May 24, 2005, at 13:36 Europe/Dublin, Maarten Buis wrote:
Dear Keith
My post would do what you want, but normal spline regression is far
far far more stable. So Nick was right (as usual) in pointing you to
-mkspline-. I would only use -nl- if the location of the knot were my
primary interest. I would still prefer to use somewhat plausible
locations (like equally spaced knots) rather than -nl-, even if I did
not know the exact location, if the exact locations of the knots are
not of substantial interest (as is usually the case). If you just want
to fit a flexible line through your data you could have a look at
-locpoly-.
Hope this helps,
Maarten
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