Refer to Exercise 15.28.
a. Plot the data in a profile plot with factors job type and region. Does there appear to be an interaction between the two factors? If there was an interaction, would you be able to test for it using the given data? If not, why not?
b. Did the geographical region variable increase the efficiency of the design over conducting the study as a completely randomized design in which the city man­ager would have randomly selected eight cities regardless of their location?
c. Identify additional sources of variability that may need to be included in future studies.
In exercise

Region Police Firefighters 39 328 36 312 30.8 306 28.7 27.5 30.64 Inspectors 27.9 278 265 268 264 268 25.3 25.9 26.68 Region mean 307313 29.6 29.5 29. 29 27.5 27.1 29.25 6 Mean 32.3 33.2 308 305 30 30.2 28.4 27.9 30.42