I have four data points:
Proportion A, N = N_A
Proportion B, N = N_B
Proportion C, N = N_C
Proportion D, N = N_D
I want to know if
(Proportion A - Proportion B) is significantly different to (Proportion C - Proportion D)
Does anyone know how you test for this?
I know how to do a normal proportions test but feel that that doesn't apply here as i'm looking for a test of the DIFFERENCE OF THE DIFFERENCE between two proportions?