Try it and tell me if it works.
MIN(Result) will take the minimum of the results.
18,2000 1 Negative 27,0000 04/06/2010
21,0000 1 Positive 9,0000 04/06/2010
14,0000 1 Same 14,0000 04/06/2010
Sorted alphabetically, the Min of (Negative, Postive and Same) is "Negative".
16,5000 2 Same 16,5000 05/06/2010
31,0000 2 Positive 20,0000 05/06/2010
21,0000 2 Same 21,0000 05/06/2010
Sorted alphabetically, the Min of (Positive and Same) is "Positive".
It will work.