# Relationship between pseudo-$R^2$ and area under the ROC curve

The $R^2$ of a model measures how well a model fits the data and is a measure of the shared variation between two (or more) variables. Its equivalent measure for logistic regression is the pseudo-$R^2$. A pseudo-$R^2$ is sometimes presented alongside the area under the receiver operator characteristic (ROC) as a measure of a model's predictive accuracy.

I'm curious as to whether there is any straightforward relationship between these two metrics. Does a model with a higher pseudo-$R^2$ necessarily have a larger AUC ROC? Are there any situations where a model can have a low pseudo-$R^2$ but a high AUC ROC? It seems intuitive that the two measures are necessarily correlated, but I've been wrong many times in the past.

## 1 Answer

The AUC is scale independant. It is solely based on ranks. If you multiply all the probabilities outputed by your logistic regression by the same factor $\lambda\in(0,1]$, the AUC will remain the same. Note that as $\lambda\rightarrow0$ the pseudo $R^2$ will decrease (possibly becoming negative).

So you can have a low pseudo $R^2$ but a large AUC.

• AUC takes some getting used to if you are accustomed to rsq or pseudo-rsq. E.g., an rsq of .05 (5% of the way from 0.00, useless prediction, to 1.00, perfect prediction) will typically be paired with an AUC near .65 (30% of the way from .50, useless prediction, to 1.00, perfect prediction). – rolando2 Oct 22 '15 at 15:52