[R] glmnet: obtain predictions using predict and also by extracting coefficients

[Weidong Gu]

Hi Juliet,

First of all, cv.glmnet is used to estimate lambda based on
cross-validation. To get a glmnet prediction, you should use glmnet
function which uses all data in the training set. Second, you
constructed testX using a different data set (data.test.std) from one
for glmnet predict (data.test). It's not surprise the predictions are
different.

Weidong Gu



On Wed, Mar 21, 2012 at 2:35 PM, Juliet Hannah <juliet.hannah at gmail.com> wrote:
> All,
>
> For my understanding, I wanted to see if I can get glmnet predictions
> using both the predict function and also by multiplying coefficients
> by the variable matrix. This is not worked out. Could anyone suggest
> where I am going wrong?
> I understand that I may not have the mean/intercept correct, but the
> scaling is also off, which suggests a bigger mistake.
>
>  Thanks for your help.
>
> Juliet Hannah
>
>
> library(ElemStatLearn)
> library(glmnet)
>
> data(prostate)
>
> # training data
> data.train <- prostate[prostate$train,]
> y <- data.train$lpsa
>
> # isolate predictors
> data.train <- as.matrix(data.train[,-c(9,10)])
>
> # test data
> data.test <- prostate[!prostate$train,]
> data.test <-  as.matrix(data.test[,-c(9,10)])
>
> # scale test data  by using means and sd from training data
>
> trainMeans <- apply(data.train,2,mean)
> trainSDs <- apply(data.train,2,sd)
>
> # create standardized test data
>
> data.test.std <- sweep(data.test, 2, trainMeans)
> data.test.std <- sweep(data.test.std, 2, trainSDs, "/")
>
> # fit training model
>
> myglmnet =cv.glmnet(data.train,y)
>
> # predictions by using predict function
>
> yhat_enet <- predict(myglmnet,newx=data.test, s="lambda.min")
>
> # attempting to get predictions by using coefficients
>
> beta  <- as.vector( t(coef(myglmnet,s="lambda.min")))
>
> testX <- cbind(1,data.test.std)
>
> yhat2  <- testX %*% beta
>
> # does not match
>
> plot(yhat2,yhat_enet)
>
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