[R-sig-Geo] Correlation between covariates and intercept (spatstat)

[Virginia Morera Pujol]

Hello all,

In trying a spatial model with spatstat I am running into a conceptual
problem. It might be more of a general modelling doubt than a specific
spatial problem, but I hope someone can help.

I am running a ppm() model that includes two covariates (as pixel images),
one is primary productivity at sea, and the other is distance to a point
that is not included in the pattern window. That means there is no 0 value,
the range of values goes from 400 to 1400 approx.  When I run the model and
look at the var-covar matrix using 'vcov(model, what = "corr")' , there is
a very strong correlation (around -0.85) between the intercept and this
covariate. I am not sure that this is a problem, but I have tried a couple
of things just in case:

1/ centering the covariate values around the mean just changes the sign of
the correlation (from -0.85 to +0.85 approx).

2/ normalizing the covariate values, so the values go from 0 to 1 makes the
correlation between this covariate and the intercept almost 1 (0.99) It
also makes the effect of this covariate three orders of magnitude higher
than the effect of the other covariate, which didn't happen before and was
not expected from the data.

Any advice? Thanks in advance!

Virginia Morera
PhD Student
Department of Animal Biology
University of Barcelona

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