Which nonparametric test should be used to test the hypothesis that variability of the two machines is comparable? 

Another aspect of the same study is to compare the variability of blood pressure with each method. This comparison is achieved by computing |x₁ˆ’ x₂| for each participant and method (i.e., absolute difference between first and second readings) and comparing absolute differences between machines for individual participants. The data are given in Table 9.14. The observers are reluctant to assume that the distributions are normal. 

Table 9.14: Comparison of variability of SBP with the standard cuff and the random-zero machine (mm Hg)

 

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Absolute difference, standard cuff (a,) Absolute difference, Person random-zero (a,) 1 2 12 4 3 4 5 4 2 4 2 10 11 12 13 14 15 16 2 17 6. 6 18 19 14 20 4