Count the number of false positives with respect to the first class Can anybody tell me the formula how to find the number of false positives with respect to the first class?



where $y$ is the truth / target and $a(x)$ is the prediction
I always find the notion of false positive and negative confusing, especially when it comes to multi-class problems.

A good rule of thumb that I came up with is the following

True positive: "I predicted that you were a certain class, and I was right"

False positive: "I predicted that you were a certain class but I was wrong"

True negative: "I predicted that you weren't a certain class, and I was right"

False negative: "I predicted that you weren't a certain class, but I was wrong"

So in your case, there are 31 true positives with regard to the first class since you predicted 31 times that something was class 1 but you were wrong.