The interaction is a function of the difference between the simple effects of A at the
two levels of B divided by two, or vice-versa.
(This works only for 2 x 2 factorials)
A x B = 1/2(Simple effect of A at b1 - Simple effect of A at b0)
= 1/2(3 - 1)
= 1
or
A x B = 1/2(Simple effect of B at a1 - Simple effect of B at a0)
= 1/2(2 - 0)
= 1