Stressed-Out Bus Drivers. An intervention program designed by the Stockholm Transit District was implemented to improve the work conditions of the city’s bus drivers. Improvements were evaluated by G. Evans et al., who collected physiological and psychological data for bus drivers who drove on the improved routes

(intervention) and for drivers who were assigned the normal routes

(control). Their findings were published in the article “Hassles on the Job: A Study of a Job Intervention with Urban Bus Drivers” (Journal of Organizational Behavior, Vol. 20, pp. 199–208). Following are data, based on the results of the study, for the heart rates, in beats per minute, of the intervention and control drivers.

Intervention Control 68 66 74 52 67 63 77 57 80 74 58 77 53 76 54 73 54 69 63 60 77 63 60 68 64 68 73 66 71 66 55 71 84 64 76 63 73 59 68 64 82

a. At the 5% significance level, do the data provide sufficient evidence to conclude that the intervention program reduces mean heart rate of urban bus drivers in Stockholm? (Note: x¯1 = 67.90, s1 = 5.49, x¯2 = 66.81, and s2 = 9.04.)

b. Can you provide an explanation for the somewhat surprising results of the study?

c. Is the study a designed experiment or an observational study? Explain your answer.