The lasso regression is based on the idea of solving\widehat{\mathbf{\beta}}_{\lambda}=\text{argmin}\lbrace -\log\mathcal{L}(\mathbf{\beta}|\mathbf{x},\mathbf{y})+\lambda\|\mathbf{\beta}\|_{\ell_1}\rbracewhere\Vert\mathbf{a} \Vert_{\ell_1}=\sum_{i=1}^d |a_i|for any \mathbf{a}\in\mathbb{R}^d. In a recent post, we’ve seen computational aspects of the optimization problem. But I went quickly throught the story of the \ell_1-norm. Because it means, somehow, that the value of \beta_1 and \beta_2 should be comparable. Somehow, with two significant variables, with very different scales, we should expect orders (or relative magnitudes) of \widehat{\beta}_1 and \widehat{\beta}_2 to be very very different. So people say that it is therefore necessary to center and reduce (or standardize) the variables.

Consider the following (simulated) dataset

Sigma = matrix(c(1,.8,.2,.8,1,.4,.2,.4,1),3,3)
n = 1000
library(mnormt)
X = rmnorm(n,rep(0,3),Sigma)
set.seed(123)
df = data.frame(X1=X[,1],X2=X[,2],X3=X[,3],X4=rnorm(n),
X5=runif(n),X6=exp(X[,3]),
X7=sample(c("A","B"),size=n,replace=TRUE,prob=c(.5,.5)),
X8=sample(c("C","D"),size=n,replace=TRUE,prob=c(.5,.5)))
df$Y = 1+df$X1-df$X4+5*(df$X7=="A")+rnorm(n)
X = model.matrix(lm(Y~.,data=df))

Use the following colors for the graphs and the value of \lambda

library("RColorBrewer")
colrs = c(brewer.pal(8,"Set1"))[c(1,4,5,2,6,3,7,8)]
vlambda=exp(seq(-8,1,length=201))

The first regression we can run is a non-standardized one

library(glmnet)
lasso = glmnet(x=X,y=df[,"Y"],family="gaussian",alpha=1,lambda=vlambda,standardize=FALSE)

We can visualize the graphs of \lambda\mapsto\widehat{\beta}_\lambda

idx = which(apply(lasso$beta,1,function(x) sum(x==0))<200)
plot(lasso,col=colrs,'lambda',xlim=c(-5.5,2.3),lwd=2)
legend(1.2,.9,legend=paste('X',0:8,sep='')[idx],col=colrs,lty=1,lwd=2)

At least, observe that the most significant variables are the one that were used to generate the data.

Now, consider the case that we standardize the data

lasso = glmnet(x=X,y=df[,"Y"],family="gaussian",alpha=1,lambda=vlambda,standardize=TRUE)

The graphs of \lambda\mapsto\widehat{\beta}_\lambda

The graph is (strangely) very similar to the previous one. Except perhaps for the green curve. Maybe that categorical are not simular to continuous variables… Because somehow, standardisation of categorical variables might be not natural…

Why not consider some home-made function ? Let us transform (linearly) all variable in the X matrix (except the first one, which is the intercept)

Xc = X
for(j in 2:ncol(X)) Xc[,j]=(Xc[,j]-mean(Xc[,j]))/sd(Xc[,j])

Now, we can run our lasso regression on that one (with the intercept since all the variables are centered, but y)

lasso = glmnet(x=Xc,y=df$Y,family="gaussian",alpha=1,intercept=TRUE,lambda=vlambda)

The plot is now

plot(lasso,col=colrs,"lambda",xlim=c(-6.7,1.3),lwd=2)
idx = which(apply(lasso$beta,1,function(x) sum(x==0))<length(vlambda))
legend(.15,.45,legend=paste('X',0:8,sep='')[idx],col=colrs,lty=1,bty="n",lwd=2)

Actually, why not also center the y variable, and remove also the intercept

Yc = (df[,"Y"]-mean(df[,"Y"]))/sd(df[,"Y"])
lasso = glmnet(x=Xc,y=Yc,family="gaussian",alpha=1,intercept=FALSE,lambda=vlambda)

Hopefully, those graphs are very consistent (and if we use those for variable selection, they suggest to use variables that were actually used to generate the dataset). And having qualitative and quantitative variable is not a big deal. But still, I do not feel confortable with the differences…