gls-crossvalidation

Dear list,

I'm trying to fit a gls model with an spatial component and I want to validate my results doing a crossvalidation. I wrote the code getting at the end the mean square error, and supposedly it should work but I'm getting huge numbers as results (ranging between 3 to 2000). Perhaps I should mention that my data have lots of zeros.
can somebody tell me what's wrong?
This is the code I'm using for the crossvalidation.

m_err2.vect<- vector()
for(j in 1:10)
    {
    print(j)
    select.rec<- sample(1:nrow(data.dmi), 0.9*nrow(data.dmi))
    train.rec<- data.rec[select.rec,]   #Selecting 90% of the data for training purpose
    test.rec<-  data.rec[-select.rec,]   #Selecting 10% (remaining) for testing purpose
    gls.rec<- gls (log(records+1)~roads+pop+conflict+airport+rails+PA+pristine+tur_plac, data = train.rec,correlation=corSpher(form=~X_Mol + Y_Mol, nugget=TRUE), na.action=na.omit)
    #Create fitted values using test.dmi data
    rec_pred<- predict(gls.rec, test.rec)
    rec_obs<-test.rec[,"records"]
    # Get the prediction error = Mean Square Error (MSE)= 1/n
    m_err2<- t(rec_pred - rec_obs)%*%(rec_pred - rec_obs)/nrow(test.rec)
    m_err2.vect<- c(m_err2.vect, m_err2)
}

  m_err2.vect

  [1]  155.68777  380.22485  121.41826 1188.19114  292.95930   40.00558  253.04283   13.58491 1239.02019  149.31290

mean(m_err2.vect)
[1] 383.3448


Thanks a lot in advance, any suggestion would be very much appreciated.

Cheers,

Liliana.





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