# Is the PR AUC invariant under label flip?

The ROC-AUC curve is invariant under a flip of the labels. I don't know if it's a famous result, so I will give the proof below. My question is if the PR-AUC curve also has this property. I have not been able to prove or disprove it yet.

The reason this is important is that when data sets are highly imbalanced ROC-AUC curves perform well mechanically and it is better to look at PR-AUC curves. This obviously raises the question of a better curve for PR-AUC for one class being better for the other class as well or not.

### Proof that ROC-AUC is invariant under label flip

$$\begin{array}{|c|c|c|} \hline & \text{Predicted positive} & \text{Predicted negative} \\ \hline \hline \text{Positives} & TP(\xi) & FN(\xi) \\ \hline \text{Negatives} & FP(\xi) & TN(\xi) \\ \hline \end{array}$$ where $$\xi \in [0,1]$$ is the output of a classifier.

For a fixed dataset we have

$$TP(\xi) + FN(\xi) = P \\ FP(\xi)+TN(\xi) = N$$ where $$P$$ and $$N$$ are actual number of negatives and positives. The ROC-AUC defined wrt to this definition of positives and negatives is

$$A = \frac{1}{PN} \int_0^1 TP(\xi)~d FP(\xi)$$

Now lets look at what happens under a label flip. We get

$$\begin{eqnarray} \tilde A &=& \frac{1}{PN} \int_1^0 TN(\xi)~d FN(\xi) \\ &=& - \frac{1}{PN} \int_1^0 (N-FP(\xi)~d TP(\xi) \\ &=& \frac{1}{PN} \int_0^1 (N-FP(\xi)~d TP(\xi) \\ &=& \frac{1}{P} TP(\xi) |_0^1 - \frac{1}{PN} (TP(\xi))(FP(\xi))|_0^1 + A \\ &=& A \end{eqnarray}$$ where the last equality follows from $$TP(0)=0$$, $$TP(1)=P$$ and $$TN(1)=N$$.