[R] logistic regression

[Dimitris Rizopoulos]

Hi Heike,

you'd  probably want

lines(xsun, predict(model2, type="response", 
newdata=data.frame(sun=xsun, qsun=xsun*xsun)))

I hope it helps.

Best,
Dimitris

----
Dimitris Rizopoulos
Ph.D. Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/16/396887
Fax: +32/16/337015
Web: http://www.med.kuleuven.ac.be/biostat/
     http://www.student.kuleuven.ac.be/~m0390867/dimitris.htm


----- Original Message ----- 
From: "Heike Zimmermann" <heikz at gmx.de>
To: <R-help at stat.math.ethz.ch>
Sent: Monday, October 11, 2004 4:32 PM
Subject: [R] logistic regression


> 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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