Dear R users;
I am modelling densities of some species of birds, so I have a problem with a
great ammount of zeros.
I have decided to try GLMs with the tweedie family, but in all the models I
have tried I got an NA for the AIC value.
Just to check the problem I've compared the a glm using the Gaussian family
with the identity link and a glm using the tweedie family with var.power=0 and
link.power=1. These are equal, as expected, except the fact that the tweedie
output gives me an NA for the AIC.
Can anyone help me with this problem?
Below you can find the two outputs I refer.
Best Wishes;
Catarina
> summary(glm(formula=ACIN~DIST_REF+DIST_H2O+DIST_OST+
> COTA+H2O_SUP+vasa,family=gaussian(link="identity")))
Call:glm(formula = ACIN ~ DIST_REF + DIST_H2O + DIST_OST + COTA + H2O_SUP +
vasa, family = gaussian(link = "identity"))
Deviance Residuals: Min 1Q Median 3Q Max
-0.112792 -0.042860 -0.021113 -0.006311 1.551824
Coefficients: Estimate Std. Error t value Pr(>|t|) (Intercept)
-6.625e-02 5.454e-02 -1.215 0.2256 DIST_REF 3.581e-06 1.336e-05
0.268 0.7889 DIST_H2O -3.168e-05 1.527e-05 -2.074 0.0391 *DIST_OST
-1.799e-05 1.953e-05 -0.921 0.3579 COTA 5.648e-04 2.470e-04
2.287 0.0230 *H2O_SUP -2.172e-04 3.994e-04 -0.544 0.5870 vasa
3.695e-02 4.573e-02 0.808 0.4199 ---Signif. codes: 0 '***' 0.001 '**'
0.01 '*' 0.05 '.' 0.1 ' ' 1
(Dispersion parameter for gaussian family taken to be 0.02151985)
Null deviance: 5.6028 on 257 degrees of freedomResidual deviance: 5.4015
on 251 degrees of freedomAIC: -249.33
Number of Fisher Scoring iterations: 2
> summary(glm(formula=ACIN~DIST_REF+DIST_H2O+DIST_OST+
> COTA+H2O_SUP+vasa,control=glm.control(maxit=750),family=tweedie(var.power=0,
> link.power=1)))
Call:glm(formula = ACIN ~ DIST_REF + DIST_H2O + DIST_OST + COTA + H2O_SUP +
vasa, family = tweedie(var.power = 0, link.power = 1), control =
glm.control(maxit = 750))
Deviance Residuals: Min 1Q Median 3Q Max
-0.112792 -0.042860 -0.021113 -0.006311 1.551824
Coefficients: Estimate Std. Error t value Pr(>|t|) (Intercept)
-6.625e-02 5.454e-02 -1.215 0.2256 DIST_REF 3.581e-06 1.336e-05
0.268 0.7889 DIST_H2O -3.168e-05 1.527e-05 -2.074 0.0391 *DIST_OST
-1.799e-05 1.953e-05 -0.921 0.3579 COTA 5.648e-04 2.470e-04
2.287 0.0230 *H2O_SUP -2.172e-04 3.994e-04 -0.544 0.5870 vasa
3.695e-02 4.573e-02 0.808 0.4199 ---Signif. codes: 0 '***' 0.001 '**'
0.01 '*' 0.05 '.' 0.1 ' ' 1
(Dispersion parameter for Tweedie family taken to be 0.02151985)
Null deviance: 5.6028 on 257 degrees of freedomResidual deviance: 5.4015
on 251 degrees of freedomAIC: NA
Number of Fisher Scoring iterations: 2
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