Consider paired binary response data. For example , suppose you have twins randomized to two treatment groups (Test and Control) then tested on a binary outcome (pass or fail). There are 4 possible outcomes for each pair : (a) both twins fail, (b) the twin in the control group fails and the one in the test group passes, (c) the twin on the test group fails and the one in the control group passes , or (d) both twins pass. Here is a table where the of the number of sets of twins falling in each of the four categories are denoted a,b,c and d: