Colleagues,

 

I have a question about interpreting coefficients produced from function ppm
(point process model) in spatstat. Say I run the following:

 

#
---------------------------------------------------------------------------

library(spatstat)

X <- rpoispp(42)

plot(X)

weirdfunction <- function(x,y){ 10 * x^2 + 5 * sin(10 * y) }

 

# Covariate values as function

ppm(X, ~ y + Z, covariates = list(Z = weirdfunction))

 

# Fitted coefficients for trend formula:

# (Intercept)           y           Z

# 4.20090449 -0.83560468 -0.07022055

#
---------------------------------------------------------------------------

 

Would the following be an appropriate interpretation of the Z coefficient?

 

"Unit increases in Z decreased the intensity of X by a factor of 0.93"
[where exp(-0.07022055) = 0.93].

 

Thanks in advance,

 

Mark S

 

 

********************************************************

Mark Stevenson

Institute of Veterinary, Animal and Biomedical Sciences

Massey University

Private Bag 11-222 Palmerston North

New Zealand

Ph: +64 (06) 350 5915

Fx: +64 (06) 350 5716

M.Stevenson@massey.ac.nz

*******************************************************

 


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