Problem 9.1, page 176 from Zar (1999)
Output from SigmaStat

Paired t-test:                      Saturday, September 30, 2000, 22:00:58

Normality Test:	Passed	(P = 0.152)

Group             N   Missing	
nitrogen oxide   11     0	
hydrocarbons     11     0	

Group            Mean    Std Dev   SEM	
nitrogen oxide  83.727   16.888   5.092	
hydrocarbons    85.818   16.443   4.958	

Difference	-2.091	4.277	1.289

t = -1.621  with  10 degrees of freedom. (P = 0.136)

 95 percent confidence interval for difference of means: -4.964 to  0.782

The change that occurred with the treatment is not great enough to exclude the
possibility that the difference is due to chance  (P = 0.136)

Power of performed test with alpha = 0.050: 0.202

The power of the performed test (0.202) is below the desired power of 0.800.
You should interpret the negative findings cautiously.