How to use the 'perfcurve' of Matlab with specific inputs?

Question

Aaronne 2013-3-19

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https://ww2.mathworks.cn/matlabcentral/answers/67789-how-to-use-the-perfcurve-of-matlab-with-specific-inputs

回答： Neil Caithness 2013-10-23

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Hi Smart guys,

I wrote following codes to get a plot of 'classification accuracy' vs. 'threshold':

(The datasets has the ground truth contains two classes labelled 'Good' or 'Bad')

    LDAClassifierObject                     = ClassificationDiscriminant.fit(featureSelcted, groundTruthGroup, 'DiscrimType', 'linear');
    [LDALabel, LDAScore]                     = resubPredict(LDAClassifierObject);
    [~, AccuracyLDA, Thr]                   = perfcurve(groundTruthNumericalLable(:,1), LDAScore(:,1), 1,'yCrit','accu');
    figure,
    plot(Thr,AccuracyLDA,'r-');
    hold on;
    plot(Thr,AccuracyLDA,'bo');
    xlabel('Threshold for ''good'' Returns');
    ylabel('Classification Accuracy');
    grid on;
    [maxVal, maxInd] = max(AccuracyLDA)
    maxVal =
        0.8696
    maxInd =
        15
    Thr(15)
    ans =
        0.7711

Also, I run the ROC analysis for the same datasets that the ground truth contains two classes labelled 'Good' or 'Bad'

    [FPR, TPR, Thr, AUC, OPTROCPT]       = perfcurve(groundTruthGroup(:,1), LDAScore(:,1), 'Good');
    OPTROCPT =
        0.1250    0.8667

Why Thr(15)=0.7711 is different from OPTROCPT(2)=0.8667 ?

Is the best cut-off point (ie, the best threshold OPTROCPT) obtained by ROC is the one has maximum accuracy of LDA?

Or maybe I am wrong, then what exactly `perfcurve(groundTruthNumericalLable(:,1), LDAScore(:,1), 1,'yCrit','accu')` tell us?

Thanks a lot.

A.

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Ilya 2013-3-19

Why would any value in Thr be equal to any value in OPTROCPT? Could you copy and paste the part in the doc that made you believe these two should be equal?

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