Air quality is measured by the concentration of a dangerous pollutant.

The mayor of a city has engaged in a program to improve traffic conditions in order to decrease the concentration ofthat pollutant. Of course, there is a lot of day-to-day variability in measures. In the past, the average concentration was 29 (measured in some units). In a sample of 20 days after completion of the program, the sample mean has been 26.9, with a standard deviation of 8.

Can we say that the mayor's program has been effective?