So I've used the Lasso method to fit a 15 predictor multiple linear regression model on the College dataset (ISLR package) with Outstate as the response variable. The problem is that the MSE value obtained is an 8 digit number which seems absurd. Using the forward stepwise method to choose a linear model with 13 predictors I obtained a 7 digit MSE value which also seems too large, but logically it should not be lower than than my Lasso model with 2 more predictors.

train.ind = sample(1:nrow(College), 0.5*nrow(College))
college.train = College[train.ind,]
college.test = College[-train.ind,]
xtrain = model.matrix(Outstate ~., college.train)[,-1]
ytrain = college.train$Outstate

xtest = model.matrix(Outstate ~., college.test)[,-1]
ytest = college.test$Outstate
cv.out = cv.glmnet(xtrain, ytrain, alpha = 1) # Fit lasso model on    training data
 bestlam = cv.out$lambda.min
lambda <- 10^seq(10, -2, length = 100)
lasso_mod = glmnet(xtrain,ytrain,alpha = 1, lambda = lambda)

lasso.pred = predict(lasso_mod,s = bestlam, newx = college.test)

MSE = mean((lasso.pred - college.test$Outstate[-2])^2)
  • $\begingroup$ Please include your code as text (using code formatting), do not just post a photograph or screenshot (see here, or just note that it is easier for us to copy-paste code than type your script (introducing errors along the way)). $\endgroup$ – Stephan Kolassa Apr 1 '20 at 10:55
  • $\begingroup$ @StephanKolassa Fixed. $\endgroup$ – Pame Apr 1 '20 at 11:06

I notice you construct a model matrix and remove the intercept. If you do not want a intercept you should specify Outstate ~0+..

Below I do the forward stepAIC:

calcMSE = function(pred,obs){mean((pred-obs)^2)}

fitAIC = stepAIC(glm(Outstate ~ 0+.,data=college.train))
AIC_MSE = calcMSE(predict(fitAIC,college.test),college.test$Outstate)

[1] 3757030

[1] 10440.67

[1] 0.1864213

You can see I get a 7 digit MSE but it's ok given that the mean of your response is ~ 10k, remember MSE is the mean square of your error, so a quick approximation shows that it's about 20% off..

I cannot reproduce so your called 8 digit MSE for lasso. Some parts of your code (for example predict(lasso..)) throws an error, so I re run something similar below:

xtrain = model.matrix(Outstate ~0+., college.train)
ytest = college.train$Outstate
cv.out = cv.glmnet(xtrain, ytrain, alpha = 1) 
bestlam = cv.out$lambda.min
lasso_mod = glmnet(xtrain,ytrain,alpha = 1, lambda =bestlam)

lasso.pred = predict(lasso_mod, 
newx = model.matrix(Outstate ~0+., college.test))

lasso_MSE = calcMSE(lasso.pred,college.test$Outstate)

[1] 3720351

You get a MSE lower than the stepAIC.


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