Suppose we do an experiment with 4 subjects.  Two subjects are randomly assigned 
to treatment A.  The other 2 receive treatment B.  The response variable is 
number right on a 10 point quiz.  The subjects that received treatment A had 
scores of 7 and 5 on the quiz.  The subjects that received treatment B had 
scores of 3 and 1 on the quiz.  Compute a 2-sided p-value for testing for a 
difference between treatment A and treatment B.


Original Data:

 A    B   Difference in Means
7 5  3 1        4

All possible assignments of subjects to treatments:

 A    B   Difference in Means
7 5  3 1        4
7 3  5 1        2
7 1  5 3        0
3 1  7 5       -4 
5 1  7 3       -2
5 3  7 1        0

2-sided p-value is 2/6 or about 0.333 because 2 of the 6 possible assignments of 
subjects to treatments have a difference as far from 0 as the difference 
observed in the original data.

There is no convincing evidence that one treatment is better than the other 
because it would not be too unusual (2 out of 6 chance) to see results as 
extreme as those we obtained even if the treatment has no effect on the quiz 
scores.