[R] Surface plot for polynomial regression
Johannes Ullrich
ullrichj at staff.uni-marburg.de
Thu Mar 31 17:50:16 CEST 2005
Dear R-experts,
my goal is to visualize the following polynomial regression as a 3D-surface:
Z = b0 + b1*X + b2*Y + b3*XY + b4*X^2 + b5*Y^2
I believe that a solution to this problem may be of interest to a wider
range of scientists because the problem is a derivative of a more general
problem, i.e.: how to describe the relationship between one dependent
variable and the DIFFERENCE between two other variables. There are numerous
problems associated with difference scores (e.g., reliability). One
suggested alternative consists of using the components of the difference
score separately in polynomial regression. So this is how I ended up with
the above regression, which is essentially a reformulation of b1*(X-Y)^2.
After consulting the help pages and archives my best guess was that the
function scatter3d could be rewritten in part to produce the desired output.
In fact, the quadratic fit output of the scatter3d function comes closest to
what I have in mind. However, I think the XY term is missing from the
quadratic fit equation. When I use wireframe to visualize the raw data,
there is a peak of the dependent variable when both X AND Y are high. Yet
this peak does not appear in the quadratic fit of scatter3d.
Any pointers would be welcome. I should add that I am not a programmer and
mainly work with high-level functions.
Thank you very much for R and for your help
Johannes
Dipl.-Psych. Johannes Ullrich
Philipps-Universität Marburg
Germany
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