st: RE: median cubic spline assumptions

[David Airey <david.airey@vanderbilt.edu>]

.

Bill Dupont recently added another .ado to do this (RC_SPLINE). A few  
others are to be found, but I like Bill's. You can use these in  
regression models to flexibly investigate (and test) linear and  
nonlinear relations. See Frank Harrell's text "Regression Modeling  
Strategies" text on the topic. I think you can choose your knot  
placement manually if you have slight gaps, rather than default knots.


> Hello statalisters,
>
> A statistics question, not really Stata related:
>
> In a recent paper, I used median cubic splines to graphically explore
> non-linearity in the relationship between two continuous variables.
> The independent variable, although continuous, was slightly clustered
> and as a result there were some gaps where no data were available. The
> reviewer said that because of this, the median cubic spline is not
> appropriate beacuse it is adapted from time series approaches and
> requires equidistant data of equal weight for meaningful
> interpretation. I did not find this prerequisite for this approach in
> any of my references, or in the Stata documentation. Has anyone else
> encountered this issue? Is the median cubic spline transformation only
> appropriate when responses are equidistant?
> Thanks for any insights, Tim
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