Hi Franzi,

Up to I know you can“t predict values without you have x2,x3 and x4 parameters. So you have three possible solution

1) set x2, x3 and x4 to Zero, *but* it will depend so much of what you want, because if you set them to zero, it means that you are adjusting something like mod<-lm(y~x1)
2) set x2,x3 and x4 to and mean value that you understand as rasonable for your purpose]
and 
3) build a set of for () looping, with the ranges of x2, x3 and x4, and see what happens with the y, for each combination of x1-x2-x3-x4.

y<-runif(101)
x1<-sample(seq(0, 100))
x2<-seq(from=0,to=100,by=1)
x3<-seq(from=0,to=100,by=1)^2
x4<-exp(seq(from=0,to=100,by=1))
df<-data.frame(cbind(y,x1,x2,x3,x4))
mod<-lm(y~x1+x2+x3+x3,data=df)

df.complete<-NULL
for (x2 in seq(from=0,to=100,by=20)) {
for (x3 in seq(from=0,to=100,by=20)) {
for (x4 in seq(from=0,to=100,by=20)) {
    x1<-seq(0, 100, by=10)
    df.new<-data.frame(cbind(x1,x2,x3,x4))
    df.new$pred<-predict(mod,new=df.new)
    df.complete<-rbind(df.complete,df.new)
}}}

head(df.complete,50)
summary(df.complete)


But you have a problema. If you have four dimensions (x1-x4), it is so hard to graph. In the case of two dimensions (like x1 and x2) you can use the function interp() function of akima“s packge to generate a 3d plot of your response variavel y-pred as a surface. 

Good luck,

miltinho
Brazil

----- Mensagem original ----
De: Fränzi Korner <fraenzi.korner@oikostat.ch>
Para: r-help@r-project.org
Enviadas: Terēa-feira, 22 de Janeiro de 2008 11:50:03
Assunto: [R] predict from a multiple regression model

Hello



how can I predict from a lm-object over a range of values of one explanatory
variable without having to specify values for all the other explanatory
variables? 



e.g. 



mod<-lm(y~x1+x2+x3+x4)



x1.new<-seq(0, 100)

predict(mod, new=list(x1=x1.new))





Here, predict() does not work, since values for x2, x3 and x4 are missing.
Is there a function or argument that, in such a case, averages or weights
over the other explanatory variables, how it is done in Genstat? 



Thanks

Fränzi





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Dr. Fränzi Korner-Nievergelt

oikostat - Statistische Analysen und Beratung

Ausserdorf 43

CH - 6218 Ettiswil

Tel.: +41 (0) 41 980 49 22

www.oikostat.ch



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Schweizerische Vogelwarte

CH - 6204 Sempach



www.vogelwarte.ch

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