# [R] ANCOVA/glm missing/ignored interaction combinations

lara harrup (IAH-P) lara.harrup at bbsrc.ac.uk
Wed Sep 3 10:46:54 CEST 2008

```Hi

I am using R version 2.7.2. on a windows XP OS and have a question
concerning an analysis of covariance with count data I am trying to do,
I will give details of a scaled down version of the analysis (as I have
more covariates and need to take account of over-dispersion etc etc) but
as I am sure it is only a simple problem but I just can't see how to fix
it.

I have a data set with count data as the response (total) and two
continuous covariates and one categorical explanatory variable (semio).
When I run the following lines, after loading the data and assigning
'semio' as a factor, taking into account that I want to consider two way
interactions:

> model.a<-glm(total~(temperature+humidity+semio)^2,family=poisson)
> summary(model.a)

I get the output below. But not all interactions are listed there are 4
semio categories, 1,2,3 and 4 but only 2,3,and 4 are listed in the
summary (semio2,semio3,semio4). And I cant for the life of me work out
why category one (semio1) is being ignored, missing etc.

Any help or suggestions would be most appreciated. Thanks in advance

Lara
lara.harrup at bbsrc.ac.uk

Call:
glm(formula = total ~ (temperature + humidity + semio)^2, family =
poisson)

Deviance Residuals:
Min       1Q   Median       3Q      Max
-22.212   -5.132   -2.484    3.200   18.793

Coefficients:
Estimate Std. Error z value Pr(>|z|)
(Intercept)          23.848754   2.621291   9.098  < 2e-16 ***
temperature          -1.038284   0.150465  -6.901 5.18e-12 ***
humidity             -0.264912   0.033928  -7.808 5.81e-15 ***
semio2               22.852664   1.291806  17.690  < 2e-16 ***
semio3                3.699901   1.349007   2.743   0.0061 **
semio4               -1.851163   1.585997  -1.167   0.2431
temperature:humidity  0.012983   0.001983   6.545 5.94e-11 ***
temperature:semio2   -0.870430   0.037602 -23.149  < 2e-16 ***
temperature:semio3   -0.060846   0.038677  -1.573   0.1157
temperature:semio4    0.055288   0.046137   1.198   0.2308
humidity:semio2      -0.114718   0.013369  -8.581  < 2e-16 ***
humidity:semio3      -0.031692   0.013794  -2.298   0.0216 *
humidity:semio4       0.008425   0.016020   0.526   0.5990
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

(Dispersion parameter for poisson family taken to be 1)

Null deviance: 10423.5  on 47  degrees of freedom Residual deviance:
2902.2  on 35  degrees of freedom
AIC: 3086.4

Number of Fisher Scoring iterations: 7

```