Display inverted ROC Curve

Question

my anomaly detection algorithm gave me an array of predictions where all the values greater than 0 should be of the positive class (= 0) and all the other should be classified as anomalies (= 1). I built my classifier as well: (I have three datasets, the one with only non-anomaly values and the other with all anomaly values):

normal = np.load('normal_score.pkl')
anom_1 = np.load('anom1_score.pkl')
anom2_ = np.load('anom2_score.pkl')

y_normal = np.asarray([0]*len(normal)) # I know they are normal
y_anom_1 = np.asarray([1]*len(anom_1)) # I know they are anomaly
y_anom_2 = np.asarray([1]*len(anom_2)) # I know they are anomaly

score = np.concatenate([normal, anom_1, anom_2])
y = np.concatenate([y_normal, y_anom_1, y_anom_2])

auc = roc_auc_score(y, score)
fpr, tpr, thresholds = roc_curve(y, score)
display = RocCurveDisplay(fpr=fpr, tpr=tpr, roc_auc=auc)

The AUC score I get is 0.02 and the plot looks like:

From what I understood this result is great because I should just reverse the labels to make it almost 0.98, but my question is: is there a way to specify it and automatically reverse it through a function? The values in my normal score data are all in the range (21;57) and the anomalies values are in the range (-1090; -1836) so it should be easy to spot them.

Answer 1

"I should just reverse the labels to make it almost 0.98"
That's not how it should be done. It is because if you can predict "normal", let's say, with 95% confidence, you can not infer from this that you can also predict "anomaly" with the same confidence. It becomes crucial in case of heavily imbalanced data which is probably the case here.

You should define which of these two you want to predict with high confidence and what are the target prediction metrics. For example, if you have a target on the precision and recall for predicting the "anomaly" then that should be your class "1" and calculate the metrics accordingly, and vice versa.