R1(1,1)
= 1/5: fraction of all positive examples that have feature 1 =
1
R1(0,1)
= 4/5: fraction of all positive examples that have feature 1 =
0
R1(1,0)
= 5/5: fraction of all negative examples that have feature 1
= 1
R1(0,0)
= 0/5: fraction of all negative examples that have feature 1
= 0