# Area under ROC curve for random forest

Does the area under ROC curve depends on which class is defined as default positive class by the random forest model?

I am using caret package in R to train and validate a random forest model.

library("ROCR")
library(caret)
rfmodel=train(x,y,method="rf",trainControl=ctrl,
ntree=500,tuneGrid=data.frame(mtry=c(2,3,4)))
print(rfmodel)
predict.rf=predict(rfmodel,testdata,type="prob")


Now predict.rf has two columns representing probability for class 0 and class 1 respectively, which of this column should be used to calculate area under ROC curve.

In current case, By default the tpr is defined by taking class 0 as the positive class. As I understand The ROC curve is a plot between tpr and fpr. Does the ROC curve and AUC change if I define the positive class as 1 and accordingly tpr and fpr will be swapped?

## 1 Answer

Yes, but it is not relevant in practice, except some very rare cases when class order is somewhat not equivalent to the model (like in one-class SVM).

Exchanging class order simply changes AUROC from $a$ to $1-a$, so anyway your model makes so much sense as AUROC is far from .5. This way it is basically safe to report $1-a$ when $a<.5$, and many AUROC implementations will do this automatically.