Question

Lead (Pb) concentrations were measured in the leachate from six sampling wells surrounding an active landfill. The landfill was closed and PB concentrations were measured one year later from the same wells. The results appear in the table below. Test the null hypothesis of no change in Pb concentration at the 5% level of significance.

Before After

1.1 7.3

6.8 6.0

5.2 5.4

8.3 7.6

7.0 6.5

7.9 7.3

Answer 1

Here we need to use paired t test.

Let d = before -after

Following table shows the calculations :

Before	After	d	(d-mean)^2
1.1	7.3	-6.2	30.991489
6.8	6	0.8	2.053489
5.2	5.4	-0.2	0.187489
8.3	7.6	0.7	1.776889
7	6.5	0.5	1.283689
7.9	7.3	0.6	1.520289
Total		-3.8	37.813334

Sample size: n= 6

Now,

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