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Topic: KNN classifier with ROC Analysis
Replies: 1   Last Post: Jun 18, 2013 11:58 AM

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Posts: 110
Registered: 6/2/11
KNN classifier with ROC Analysis
Posted: Mar 19, 2013 8:30 AM
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Hi Smart guys,

I wrote following codes to get a plot of ROC for my KNN classifier:

load fisheriris;

features = meas;
featureSelcted = features;
numFeatures = size(meas,1);

%% Define ground truth
groundTruthGroup = species;

%% Construct a KNN classifier
KNNClassifierObject =, groundTruthGroup, 'NumNeighbors', 3, 'Distance', 'euclidean');

% Predict resubstitution response of k-nearest neighbor classifier
[KNNLabel, KNNScore] = resubPredict(KNNClassifierObject);

% Fit probabilities for scores
groundTruthNumericalLable = [ones(50,1); zeros(50,1); -1.*ones(50,1)];
[FPR, TPR, Thr, AUC, OPTROCPT] = perfcurve(groundTruthNumericalLable(:,1), KNNScore(:,1), 1);

Then we can plot the FPR vs TPR to get the ROC curve.

However, the FPR and TPR is different from what I got using my own implementation that the one above will not display all the points, actually, the codes above display only three points on the ROC. The codes I implemented will dispaly 151 points on the ROC as the size of the data is 150.

patternsKNN = [KNNScore(:,1), groundTruthNumericalLable(:,1)];
patternsKNN = sortrows(patternsKNN, -1);
groundTruthPattern = patternsKNN(:,2);

POS = cumsum(groundTruthPattern==1);
TPR = POS/sum(groundTruthPattern==1);
NEG = cumsum(groundTruthPattern==0);
FPR = NEG/sum(groundTruthPattern==0);

FPR = [0; FPR];
TPR = [0; TPR];

May I ask how to tune '`perfcurve`' to let it output all the points for the ROC? Thanks a lot.


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