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Asked by Nirmal
on 4 Jun 2012

I am trying to use Naive Bayes for some classification task, I am not sure what it is complaining about.

??? Error using ==> NaiveBayes.fit>gaussianFit at 535 The within-class variance in each feature of TRAINING must be positive. The within-class variance in feature 5 6 12 13 15 16 17 in class 1 are not positive.

Error in ==> NaiveBayes.fit at 498 obj = gaussianFit(obj, training, gindex);

Thank you for reading

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Answer by Tom Lane
on 5 Jun 2012

Accepted answer

Suppose you have data X and classes C. Can you look at

var(X(C==1,:)

If you see that columns 5, 6, 12, etc. have zero variance, that is the problem. The fit is based on fitting a normal distribution separately for each class and feature. If any class has 0 variance for a feature, that normal fit is degenerate.

What you want to do about this depends on you. It is possible to change the fit to a kernel density estimate and specify the width. Or you could try a decision tree or knn classifier.

## 1 Comment

## the cyclist (view profile)

Direct link to this comment:http://uk.mathworks.com/matlabcentral/answers/40289#comment_83063

Are you able to post a small bit of your data and code that exhibit the error?