RDA and trend surface regression

Jari Oksanen · 2007-02-27T07:27:08Z

> 'm performing RDA on plant presence/absence data, constrained by > geographical locations. I'd like to constrain the RDA by the "extended > matrix of geographical coordinates" -ie the matrix of geographical > coordinates completed by adding all terms of a cubic trend surface > regression- . > > This is the command I use (package vegan): > > > > >rda(Helling ~ x+y+x*y+x^2+y^2+x*y^2+y*x^2+x^3+y^3) > > > > where Helling is the matrix of Hellinger-transformed presence/absence data > >

Jari Oksanen

Mon, Feb 26, 2007 11:27 PM

You must *I*solate the polynomial terms with function I ("AsIs") so that
they are not interpreted as formula operators:

rda(Helling ~ x + y + I(x*y) + I(x^2) + I(y^2) + I(x*y^2) + I(y*x^2) +
I(x^3) + I(y^3))

If you don't have the interaction terms, then it is easier and better
(numerically) to use poly():

rda(Helling ~ poly(x, 3) + poly(y, 3))

Another issue is that in my opinion using polynomial constraints is an
Extremely Bad Idea(TM).

cheers, Jari Oksanen

Thread (6 messages)

Jari Oksanen RDA and trend surface regression Feb 26 MORLON RDA and trend surface regression Feb 27 Kuhn, Max RDA and trend surface regression Feb 27 Gavin Simpson RDA and trend surface regression Feb 27 Roger Bivand RDA and trend surface regression Feb 27 Jari Oksanen RDA and trend surface regression Feb 27