*************************************************************************
* This is an example of an analysis of the data from a 2x2 crossover    *
* for a binary outcome, assuming nonnull period effects.                *
*************************************************************************;

proc format;
value outfmt 0='Failure' 1='Success';
value preffmt 1='A' -1='B';
run;

data example;
input patient sequence $ treatment_A treatment_B;
cards;
01 AB 0 0
02 AB 1 0
03 AB 0 0
04 AB 0 1
05 AB 1 0
06 AB 0 0
07 AB 0 0
08 AB 0 0
09 AB 1 1
10 AB 0 1
11 AB 0 0
12 AB 1 0
13 AB 0 0
14 AB 0 0
15 AB 0 1
16 AB 0 0
17 AB 1 1
18 AB 0 1
19 AB 1 1
20 AB 0 1
21 AB 1 1
22 AB 0 1
23 AB 0 0
24 AB 1 1
25 AB 0 1
26 BA 0 1
27 BA 0 0
28 BA 0 0
29 BA 1 0
30 BA 0 1
31 BA 0 0
32 BA 0 1
33 BA 1 0
34 BA 0 1
35 BA 0 0
36 BA 1 0
37 BA 0 0
38 BA 0 1
39 BA 0 0
40 BA 0 1
41 BA 1 0
42 BA 0 1
43 BA 0 0
44 BA 1 1
45 BA 0 0
46 BA 0 1
47 BA 0 0
48 BA 1 1
49 BA 0 0
50 BA 0 0
;
run;

data example2;
set example;
preference=treatment_A - treatment_B;
if preference=0 then delete;
format preference preffmt.; 
if sequence='AB' & preference=1 then period2=0;
if sequence='AB' & preference=-1 then period2=1;
if sequence='BA' & preference=1 then period2=1;
if sequence='BA' & preference=-1 then period2=0;
run;

proc logistic data=example2;
model preference=period2;
exact 'intercept' intercept;
exact 'period2' period2;
title "Logistic Regression Analysis for a Binary Outcome in a 2 x 2 Crossover Trial";
run;