Problem 9.2, page 177 from Zar (1999)
Output from SigmaStat

Wilcoxon Signed Rank Test                    Saturday, September 30, 2000, 22:01:48

Data source: Data 1 in Notebook

Normality Test:	Passed	(P = 0.152)

Group             N    Missing	
nitrogen oxide   11      0	
hydrocarbons     11      0	
Tested           11      0	

Group           Median     25%      75%	
nitrogen oxide  84.000   73.250   93.750	
hydrocarbons    83.000   72.250   94.250	

W= 34.000  T+ = 50.000  T-= -16.000  P(est.)= 0.142  P(exact)= 0.147

The change that occurred with the treatment is not great enough to exclude
the possibility that it is due to chance  (P = 0.147).