I'm evaluating a classifier to detect objects.
I have a set1 with 50 pictures with the object to detect.
And set2 with 100 pictures without the object.
I determine the true positves (0 in set2), true negatives (0 in set1), false negatives and false positives for those sets.
If I want to merge set1 with set2 (to have a set with and without objects and real values for TP and TP) I have to decrease the size of set2.
Can I do this with a simple division? (divide the values of TN, TP etc by 2) or do I have to randomly pick 50 pictures of set2 and use these TN's etc.?
Set1: TN = 43 FP = 6 FN = 1 TP = 0 Set2: TN = 0 FP = 13 FN = 0 TP = 87 Set2 divided by 2: TN = 0 FP = 7 FN = 0 TP = 44 Set12: TN = 43 FP = 13 FN = 1 TP = 44
I need an explanation to back this up. thx..