How to calculate FAR FRR Accuray using confusion matrix with more than 3 classes?

Question

I have a 20x20 confusion matrix. How can I calculate FAR, FRR, Accuracy and Precision values which represent complete system? Not for each class, but for the whole system.

Answer 1

One simple approach might be to divide the sum of the diagonal elements by sum of all elements in the matrix.

That would be, for example, a generalisation of accuracy.

For example, precision might be calculated by taking the average of precision per class (PPC), which you get by dividing the diagonal element in a row by the row-sum and then averaging these PPCs for each row.