Qn regarding Mahalanobis distance

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Hello,

I am writing a simple function to compute the mahalanobis distance from the mean.

pow(input, 2, input);
divide(input, variance, out);

cout << sum(out)(0) << endl;

Based on what I read, the distance should be within 3 standard deviations or in this case, sum(out)(0) should be lower than 9. However I am getting values like 20+. Could it be because my training data is not good ?