One thing I notice immediately is a number of NA values for your
coefficients. If I were you, I would try a model with less parameters, and
use the anova() function to compare models, to see if the extra terms really
improve the model.
e.g.
fit1 <- glm(Y~X1+X2+X3,...)
fit2 <- glm(Y~X1+X2+X3+X1:X2,...)
anova(fit1, fit2, test="F")

If you checked all these, understanding the interaction terms will be most
easy if you normalized your numeric data before the analysis. For the
interpretations, you just fill in some values to get an idea. For example :

given the model : Y= a+b1*X1+b2*X2+b3*X1*X2

Say X1 and X2 are numeric :
interpretation of the main term : "Y increases with b2 for an increase of 1
unit in X2, given X1 is average".
interpretation of the interaction term : "For an X1 value of n units from
the mean, X2 increases with b2+n*b3" (n is negative when value is lower than
the mean).
In a Y ~ X2 plot, you can make this visible by plotting 3 different
functions : one for a low X1 value, one for an average X1 value and one for
a high X1 value. This gives you an indication of the effect of X1 on X2.

for an interaction between a categorical terms or a categorical and a
numerical, you follow exact the same reasoning, but keep in mind that the
reference level represents a 0, and the mentioned level represents a 1. Fill
in the values in the equation, and you can understand the meaning of the
terms. Then again, you can plot a seperate function Y~X1 for every level of
a certain factor.

This isn't a straight answer on your question, but I'm afraid there is none.
I hope this helps you with building your model.

Kind regards.
Joris

On Fri, Mar 6, 2009 at 11:04 PM, Pablo Pita Orduna <ppita@udc.es> wrote:

> Hi all,
>
> I“m fitting GLM“s and I can“t interprete the coefficients when I run a
> model with interaction terms.
>
> When I run the simpliest model there is no problem:
>
> Model1<-glm (Fishes ~ Year + I(Year^2) + Kind.Geographic + Kind.Fishers +
> Zone.2 + Hours + Fishers + Month, family = poisson(log)) # Fishes, Year,
> Hours, and Fishers are numeric, Kind.Geographic, Kind.Fishers, Zone.2 and
> Month are factors with 4, 3, 5 and 12 levels respectively.
>
> Model1$coefficients (whith Helmert contrasts):
>
>   (Intercept)             Year           IYear^2 Kind.Geographic1
> Kind.Geographic2 Kind.Geographic3    Kind.Fishers1    Kind.Fishers2
>  Zone.21          Zone.22          Zone.23          Zone.24
>  -4.416915e+02     4.758455e-01    -1.270986e-04    -5.436199e-01
>  -1.068809e-01    -1.498580e-01     2.958462e-01     1.316589e-01
>  -1.328204e-01    -1.605802e-01     5.281869e-03     7.422885e-02
>         Hours          Fishers           Month1           Month2
> Month3           Month4           Month5           Month6         Month7
>       Month8           Month9          Month10
>  9.772076e-02    -2.709955e-03    -1.586887e-01    -1.887837e-02
>  -5.183241e-03     5.870942e-02     7.075386e-02     2.061223e-02
> 7.372268e-03    -1.204835e-02    -5.047994e-03     2.441498e-02
>       Month11
>  -5.665261e-03
>
> So I can write, for example:
>
> y = -4.416915e+02 + -1.270986e-04*x^2 + 4.758455e-01*x # And add this
> function to a plot(Year,Fishes).
>
> My problem is to understand the coefficients for the model with
> interaction:
>
> Model2<-glm(Fishes ~ Year + I(Year^2) + Kind.Geographic + Kind.Fishers +
> Zone.2 + Hours + Fishers + Month + Year:Kind.Geographic + Year:Kind.Fishers
> + Year:Zone.2 + Year:Hours + Year:Fishers + Year:Month +
> Kind.Geographic:Hours + Kind.Fishers:Hours + Zone.2:Hours + Hours:Fishers +
> Hours:Month +Kind.Geographic:Fishers + Zone.2:Fishers + Fishers:Month ,
> poisson (log))
>
> Model2$coefficients (with Helmert contrast):
>
>           (Intercept)                     Year                I(Year^2)
>     Kind.Geographic1         Kind.Geographic2         Kind.Geographic3
>      Kind.Fishers1            Kind.Fishers2
>          1.641473e+03            -1.748703e+00             4.664752e-04
>        -6.721427e+00             1.856033e+01          -3.762727e-02
>     2.903564e+01             9.022858e+01
>               Zone.21                  Zone.22                  Zone.23
>              Zone.24                    Hours                  Fishers
>             Month1                   Month2
>          8.110814e-02            -1.902803e+01             8.335792e+00
>        -3.661641e+00            -7.824623e+00          7.088065e-01
>     2.479387e+03             8.346729e+02
>                Month3                   Month4                   Month5
>               Month6                   Month7                Month8
>           Month9                  Month10
>          4.052680e+02             2.384440e+02             1.570644e+02
>         1.032445e+02             7.930499e+01          6.487925e+01
>     5.592869e+01             3.888328e+01
>               Month11    Year:Kind.Geographic1    Year:Kind.Geographic2
>  Year:Kind.Geographic3       Year:Kind.Fishers1       Year:Kind.Fishers2
>         Year:Zone.21          Year:Zone.22
>          4.801656e+01             3.397984e-03            -9.443234e-03
>                   NA            -1.449305e-02            -4.470212e-02
>      -6.269309e-05             9.421045e-03
>          Year:Zone.23             Year:Zone.24               Year:Hours
>         Year:Fishers              Year:Month1            Year:Month2
>      Year:Month3              Year:Month4
>         -4.184866e-03             1.854810e-03             3.257250e-03
>        -4.103058e-04            -1.264934e+00          -4.255907e-01
>    -2.069909e-01            -1.216459e-01
>           Year:Month5              Year:Month6              Year:Month7
>          Year:Month8              Year:Month9             Year:Month10
>       Year:Month11   Kind.Geographic1:Hours
>         -8.015823e-02            -5.278291e-02            -4.054404e-02
>        -3.313487e-02            -2.846036e-02            -1.973118e-02
>      -2.410902e-02             1.341231e-01
>  Kind.Geographic2:Hours   Kind.Geographic3:Hours      Kind.Fishers1:Hours
>    Kind.Fishers2:Hours            Zone.21:Hours            Zone.22:Hours
>        Zone.23:Hours            Zone.24:Hours
>          5.806418e-02                       NA             1.318444e-02
>        -1.234521e-01             7.961319e-04          1.622411e-02
>    -5.357266e-04             7.749412e-03
>         Hours:Fishers             Hours:Month1             Hours:Month2
>         Hours:Month3             Hours:Month4          Hours:Month5
>     Hours:Month6             Hours:Month7
>         -6.805803e-03             7.971549e+00             2.627090e+00
>         1.403430e+00             7.747360e-01          5.163810e-01
>     3.732549e-01             2.827511e-01
>          Hours:Month8             Hours:Month9            Hours:Month10
>        Hours:Month11 Kind.Geographic1:Fishers Kind.Geographic2:Fishers
> Kind.Geographic3:Fishers          Zone.21:Fishers
>          2.091230e-01             1.057001e-01             1.159929e-01
>                   NA            -3.422994e-02          -3.421851e-03
>               NA            -3.802354e-03
>       Zone.22:Fishers          Zone.23:Fishers          Zone.24:Fishers
>       Fishers:Month1           Fishers:Month2           Fishers:Month3
>     Fishers:Month4           Fishers:Month5
>          1.618358e-05             4.197350e-04            -8.773088e-05
>         7.806815e-01             2.509529e-01             1.280752e-01
>       7.520139e-02             5.370220e-02
>        Fishers:Month6           Fishers:Month7           Fishers:Month8
>       Fishers:Month9          Fishers:Month10        Fishers:Month11
>          3.786759e-02             2.936135e-02             2.381810e-02
>         2.422438e-02                       NA                    NA
>
>
> I would like to know if it is possible to extract the intercepts and the
> slopes for Year and Year^2 for this Model2.
>
> Thank you very much for your help.
>
> ________________________________________________________________________________
> Pablo Pita Orduna
> Grupo de Recursos Marinos y Pesquerķas.
> Universidade de A Coruńa. Campus da Zapateira s/n. E-15071. A Coruńa,
> Spain.
> Tel. +34(981) 167000 ext. 2204 Fax. +34(981) 167065.
> www.fismare.net/verdeprofundo
>
>
>
>
>
> ----------------------------------------------------------------
> Correo enviado usando el servicio de Webmail de la UDC.
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

	[[alternative HTML version deleted]]


