accuracy on a matrix

[Jaroslav Hajek]

On Tue, Jun 2, 2009 at 2:40 AM, Carlo Rossi <serosole at yahoo.it> wrote:
> Hello,
>  I have a problem that is not strictly on Octave but maybe it can be
> interesting as I didn't find solution anywhere.
> I have a matrix where each column/rows represent a class; I'm speaking about
> a confusion matrix.
> for example, three classes conf. matrix
> A = [2 1 1; 0 3 1; 0 0 4];
>
> and I read this: http://en.wikipedia.org/wiki/Accuracy_and_precision
> Is there any chance to use the first formula of accuracy (actually with more
> than 2 classes I don't understand how apply it) without use the
> Prevalence,Sensitivity etc?
>
> thanks,
>

It's obvious, isn't it?
accuracy = trace(A) / sum(A(:));
Diagonal elements represent correct classifications, the rest are
misclassifications.

cheers

-- 
RNDr. Jaroslav Hajek
computing expert & GNU Octave developer
Aeronautical Research and Test Institute (VZLU)
Prague, Czech Republic
url: www.highegg.matfyz.cz