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Thread: Why am I getting a 1.000 ROC area value even when I don't have 100% of accuracy?

  1. #1
    Join Date
    Aug 2016
    Posts
    4

    Default Why am I getting a 1.000 ROC area value even when I don't have 100% of accuracy?

    Sorry if this issue has been posted before, but I couldn't find it

    I am using Weka as a classifier, and it has worked great for me so far. However, in my last test, I got a 1.000 ROC area value (which, if i remember correctly, represents a perfect classification) without having 100% of accuracy, as can be seen in the Confusion Matrix in the Figure.


    My question is: Am I interpreting the results incorrectly or am I getting wrong results (maybe the classifier I am using is badly programmed, although I don't think it's likely)?


    I've tested it in versions 3.6 and 3.8 and had the same problem in both of them.

    Thank you!

  2. #2
    Join Date
    Aug 2006
    Posts
    1,741

    Default

    ROC is a summary statistic that measures the ranking performance of a classifier. Accuracy is just one point on the ROC curve - the point that corresponds to a threshold of 0.5 on the probability assigned to the positive class (in a two class problem). If you imagine a classifier that always ranks the positive class higher than the negative one, when sorting predictions according to the probability assigned to the positive class, then this classifier will have an AUC of 1.0. If some negative instances receive a predicted probability of being positive that is greater than 0.5 (but still less than the probability assigned to true positive instances) then this will result in misclassification when using a threshold of 0.5.

    Cheers,
    Mark.

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