An instrument called a "Visiplume" measures ultraviolet light. By comparing absorption in clear air and absorption in polluted air, the concentration of SO2 in the polluted air can be estimated. The EPA has a standard method for measuring SO2, and we wish to compare the two methods across a range of air samples. The recorded response is the ratio of the Visiplume reading to the EPA standard reading. The four experimental conditions are: measurements of SO2 in an inflated bag (n = 9), measurements of a smoke generator with SO2 injected (n = 11), measurements at two coal-fired plants (n = 5 and 6). We are interested in whether the Visiplume instrument performs the same relative to the standard method across all experimental conditions, between the coal-fired plants, and between the generated smoke and the real coal-fired smoke. The data follow (McElhoe and Conner 1986):
