reported drinking water arsenic concentrations in parts per billion ppb for 10

Reported drinking water arsenic concentrations in

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reported drinking water arsenic concentrations in parts per billion (ppb) for 10 metropolitan Phoenix communities and 10 communities in rural Arizona. The following summary values were reported: 1 x = 12.5; 2 x = 27.5; s 1 = 7.63; and s 2 = 15.3. We wish to determine whether any difference exists in mean arsenic concentrations for metropolitan Phoenix communities and for communities in rural Arizona. Figure 10-3 shows a normal probability plot for the two samples of arsenic concentration. 3 Montgomery & Runger, 2014, 387
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Inference for Two Samples 10 2.3. CI on the Difference in Means, Variances Unknown Case 1: 2 2 1 2 σ σ = The CI for the difference in means when both variances are equal can be developed from Eqn. 10-13. Case 2: 2 2 1 2 σ σ The CI for the difference in means when the assumption that 2 2 1 2 σ σ = is unwarranted can be developed from the sampling distribution of T*, ( ) 1 2 1 2 * 2 2 1 2 1 2 µ µ = + X X T S S n n (Montgomery and Runger, 2014, 383- 392) .
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