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.

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#### Best 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 $lambdain(0,1]$, the **AUC** will remain the same. Note that as $lambdarightarrow0$ the pseudo $R^2$ will decrease (possibly becoming negative).

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

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