I was reading an answer in qoura to calculate the specificity of a 3 class classifier from a confusion matrix. In the below answer
For below 3-class confusion matrix, The below is a screenshot from the answer.
the sensitivity and specificity would be found by calculating the following:
My question is in numerator for specificity which is false negatives shouldnt it be 4 terms. Eg if we are calculating w/r 1, Then in the table n22,n33,n32 and n23 were all got predicted as negatives(as they are not 1). And they are true too.
So shouldnt the formula be
specificity(w/r) Class 1 = n22+n33+n32+n23/(n22+n33+n32+n23+n31+n21)