Re: [R] glm predict on new data

From: Tóth Dénes <tdenes_at_cogpsyphy.hu>
Date: Thu, 07 Apr 2011 00:24:06 +0200 (CEST)

Dear Dirk,

You should avoid indexing in the glm call so that the name of the terms will not contain the indexing part. (Check str(lg) in your example.) A more preferred solution uses predefined data frames in the original calls: n <- 250
x <- rnorm(n)
noise <- rnorm(n,0,0.3)
y <- round(exp(x+noise)/(1+exp(x+noise)),digits=0) datfr <- data.frame(x=x,y=y)
lg <- glm(y~x,data=datfr[1:200,],family="binomial") pred <- predict(lg,newdata=datfr[201:n,],type="response")

HTH,
  Denes

> I am aware this has been asked before but I could not find a resolution.
>
> I am doing a logit
>
> lg <- glm(y[1:200] ~ x[1:200,1],family=binomial)
>
> Then I want to predict a new set
>
> pred <- predict(lg,x[201:250,1],type="response")
>
> But I get varying error messages or warnings about the different number of
> rows. I have tried data/newdata and also to wrap in data.frame() but
> cannot
> get to work.
>
> Help would be appreciated.
>
> Dirk.
>
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