e effectively without assistance was 13 years old. Among all of those patients who could use the mechanical device effectively, all but one preferred the mechanical device to the manual method of chest percussion. With one exception also, the parents of the subjects preferred the use of the mechanical device to the manual method. Based on sputum measurements, the researchers concluded that the mechanical device was as effective as the manual methods of chest percussion (Maxwell & Redmond, 1979).
The researchers exercised sufficient care in the design and implementation of the test to minimize the potential for test bias. The research sample, however, was of some concern. First, the sample was quite small. Second, and of greater importance, the research sample was not selected through the application of random procedures. This absence of randomness in the selection of the sample precluded any generalization of the test findings to a population of subjects wider than the test sample subjects.
Pryor, Parker, and Webber (1981) reported on a test of the Salford percussor two years subsequent to the Maxwell and Redmond (1979) study. As was true of the Equi-Med
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