Mathematics: Writing down a three-class classifier confusion matrix

Question

Confusion matrix 2A three-class classifier is evaluated on a test set of 900 samples which containsall three classes in equal proportions.

• Classes 2 and 3 are always classified correctly
• Class 1 is confused with class 2 in 50% of the cases, 
  and with class 3 in 20% of the cases.

Write down the confusion matrix!

Like the name "confusion matrix" already says, I'm confused. I'm using a binary classification confusion matrix which looks like that

I show you this matrix, because there are a lot of different looking classification matrices with a different order of the true positives, true negatives and so on.

According to this task, I must use a three-class classifier matrix. So first I divided the 900 Samples to 3, so I get 300 for each class. Then I substracted 50% and 20% of 300 to get 90 samples for the first class.

I read, the row index is for the true class and the column index is, what the classifier says.

My confusion Matrix looks like this:

I think, the confusions of class 1 are false positives. Did I write the matrix correctly?

Answer 1

No, you did it incorrectly, you need to switch rows with columns (transpose). Next time, please check if a similar question has not been asked before. For the explanation, see https://stats.stackexchange.com/questions/318780/how-to-understand-confusion-matrix-for-3x3 .