[R] negetative AIC values: How to compare models with negative AIC's

From: Jan Verbesselt <Jan.Verbesselt_at_biw.kuleuven.be>
Date: Sat 16 Apr 2005 - 00:43:31 EST

Dear,

When fitting the following model
knots <- 5
lrm.NDWI <- lrm(m.arson ~ rcs(NDWI,knots)

I obtain the following result:

Logistic Regression Model

lrm(formula = m.arson ~ rcs(NDWI, knots))

Frequencies of Responses
  0 1
666 35

       Obs  Max Deriv Model L.R.       d.f.          P          C        Dxy
Gamma      Tau-a         R2      Brier 
       701      5e-07      34.49          4          0      0.777      0.553
0.563      0.053      0.147      0.045 

          Coef     S.E.    Wald Z P     
Intercept   -4.627   3.188 -1.45  0.1467
NDWI         5.333  20.724  0.26  0.7969
NDWI'        6.832  74.201  0.09  0.9266
NDWI''      10.469 183.915  0.06  0.9546
NDWI''' -190.566 254.590 -0.75 0.4541 When analysing the glm fit of the same model

Call: glm(formula = m.arson ~ rcs(NDWI, knots), x = T, y = T)

Coefficients:

            (Intercept) rcs(NDWI, knots)NDWI rcs(NDWI, knots)NDWI' rcs(NDWI, knots)NDWI'' rcs(NDWI, knots)NDWI'''

                0.02067                  0.08441                 -0.54307
3.99550                -17.38573  

Degrees of Freedom: 700 Total (i.e. Null);  696 Residual
Null Deviance:      33.25 
Residual Deviance: 31.76        AIC: -167.7 

A negative AIC occurs!

How can the negative AIC from different models be compared with each other? Is this result logical? Is the lowest AIC still correct?

Thanks,
Jan



ir. Jan Verbesselt
Research Associate
Lab of Geomatics Engineering K.U. Leuven Vital Decosterstraat 102. B-3000 Leuven Belgium Tel: +32-16-329750 Fax: +32-16-329760
http://gloveg.kuleuven.ac.be/

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https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sat Apr 16 00:47:56 2005

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