[R] Prediction using GAM

From: Kerry Bush <kerryrekky_at_yahoo.com>
Date: Thu 24 Mar 2005 - 15:31:23 EST

Recently I was using GAM and couldn't help noticing the following incoherence in prediction:

> data(gam.data)
> data(gam.newdata)

> gam.object <- gam(y ~ s(x,6) + z, data=gam.data)
> predict(gam.object)[1]

        1
0.8017407
>

predict(gam.object,data.frame(x=gam.data$x[1],z=gam.data$z[1]))

        1
0.1668452

I would expect that using two types of predict arguments should give me the same results. When I used this to predict a new data set then it seems OK:

>

predict(gam.object,data.frame(x=gam.newdata$x[1],z=gam.newdata$z[1]))

        1
0.4832136
> predict(gam.object,gam.newdata)[1]

        1
0.4832136

Could anybody explain the strange behavior of predict.gam function?

Thanks,
Kai



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