# st: Re: spline regression (Kit Baum)

 From Kit Baum To statalist@hsphsun2.harvard.edu Subject st: Re: spline regression (Kit Baum) Date Tue, 25 Mar 2008 06:48:22 -0400

Mohammed,

If you graph x vs y, and break the line at the knot points, a linear spline allows the line to have kinks, like a dot-to-dot drawing. A quadratic spline has constant first derivatives == slopes at the knot points, so that it will not have any kinks. I don't know how to explain a second derivative in this context except to say that a curved line may have more or less curvature (such as a railroad track on a curve may be a broad curve or a sharp curve) and holding the second derivative constant causes the degree of curvature to be equal before and after the knot point (so that the locomotive will not derail at the knot point).

Kit Baum, Boston College Economics and DIW Berlin
http://ideas.repec.org/e/pba1.html
An Introduction to Modern Econometrics Using Stata:
http://www.stata-press.com/books/imeus.html

On Mar 24, 2008, at 02:33 , Mohammed wrote:

```Thank you very much. Pardon me, I am not good in MAth.
i will be very grateful if you explain more what you
mean by  "the derivative (slope) of the function is
equal on either side of each knot point, but the
curvature on either side may differ" and "The first
and  second derivatives of the function are equal on
either side of each  knot point." Thanks again Kit
```
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