[R] logistic regression
Heike Zimmermann
heikz at gmx.de
Mon Oct 11 16:32:51 CEST 2004
Hello,
I have a problem concerning logistic regressions. When I add a quadratic
term to my linear model, I cannot draw the line through my scatterplot
anymore, which is no problem without the quadratic term.
In this example my binary response variable is "incidence", the explanatory
variable is "sun":
> model0<-glm(incidence~1,binomial)
> model1<-glm(incidence~sun,binomial)
> anova(model0,model1,test="Chi")
Analysis of Deviance Table
Model 1: incidence ~ 1
Model 2: incidence ~ sun
Resid. Df Resid. Dev Df Deviance P(>|Chi|)
1 299 332.94
2 298 287.19 1 45.74 1.349e-11
> qsun<-sun^2
> model2<-glm(incidence~sun+qsun,binomial)
> anova(model1,model2,test="Chi")
Analysis of Deviance Table
Model 1: incidence ~ sun
Model 2: incidence ~ sun + qsun
Resid. Df Resid. Dev Df Deviance P(>|Chi|)
1 298 287.194
2 297 280.623 1 6.571 0.010
So the second, non-linear, model explains more than the first.
Now to create a graph I write:
> plot(sun,incidence)
> min(sun)
[1] 0
> max(sun)
[1] 90
> xsun<-seq(0,90,1)
>lines(xsun,predict(model2,type="response",data.frame(sun=xsun)))
Error in model.frame(formula, rownames, variables, varnames, extras,
extranames, :
variable lengths differ
>
So this is the message I receive everytime I try to draw the fitted values
of my model. I know for sure that exactly the same command works in S-Plus
(with the same data). How is ist possible to do this in R?
Thank you in advance, Heike
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