9.46 One-hour carbon monoxide concentrations in 45 air samples from a section of a city showed an average of 11.6 ppm and a variance of 82.4. After a traffic control strategy was put into place, 19 air samples showed an average carbon monoxide concentration of 6.2 ppm and a variance of 38.1.

Estimate the true difference in average carbon monoxide concentrations in a 95% confidence interval. What assumptions are necessary for your answer to be valid? (See Zamurs, Air Pollution Control Association Journal, 34, no. 6, 1984, p. 637, for details of the measurements.)