GMACRO
RD
Random 500 C1;
Normal 50 5.0.
Random 500 C2;
Normal  0  5.0.
Random 500 C3;
Normal  0  5.0.
Add C1 C2 C4
Code (0:50) 1 C4 C5
Code (50:100) 0 C5 C5
Table C5
Let C6= C1 + C3 + (10*C5)
Name  C1 = 'true'  C2 ='x error'   C3 ='y error'  C4 ='pretest'   C5='group'  C6='posttest' 
Table C5;
means C4 C6. 
Plot C6  * C4;
Symbol C5.
Let C7 = C4 - 50
Name C7 = 'pre-cut'
Let C8 = C7 * C5
Let C9 = C7 * C7
Let C10 = C9 * C5
Name C8 ='I1'  C9 = 'pre2' C10 = 'I2' 
Brief 1
Regress C6 2 C7 C5
Regress C6 3 C7 C5 C8
Regress C6 4 C7 C5 C8 C9 
Regress C6 5 C7 C5 C8 C9 C10
ENDMACRO