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Trace metals in drinking water affect the flavor and an unusually high concentration can pose a health hazard. Ten pairs of data were taken measuring zinc concentration in bottom water and surface water of a water source. Location Zinc concentration in bottom water Zinc concentration in surface water 1 .430 .415 2 .266 .238 3 .567 .390 4 .531 .410 5 .707 .605 6 .716 .609 7 .651 .632 8 .589 .523 9 .469 .411 10 .723 .612 Do the data support that the zinc concentration is less on the bottom than the surface of the water source, at the α = 0.1 level of significance? Note: A normal probability plot of difference in zinc concentration between the bottom and surface of water indicates the population could be normal and a boxplot indicated no outliers.

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Answer:

There is enough evidence to support the claim that the zinc concentration is higher on the bottom than the surface of the water source.

Step-by-step explanation:

We have the data

Zinc conc. bottom water (X)  | Zinc conc. in surface water (Y)

1 .430 .415

2 .266 .238

3 .567 .390

4 .531 .410

5 .707 .605

6 .716 .609

7 .651 .632

8 .589 .523

9 .469 .411

10 .723 .612

We can calculate the difference Di=(Xi-Yi) for each pair and calculate the mean and standard deviation of D.

If we calculate Di for each pair, we get the sample:

D=[0.015 0.028 0.177 0.121 0.102 0.107 0.019 0.066 0.058 0.111 ]

This sample, of size n=10, has a mean M=0.0804 and a standard deviation s=0.0523.

All the values are positive, what shows that the concentration water appearse to be higher than the concentration on the bottom.

We can test this with a t-model.

The claim is that the zinc concentration is greater on the bottom than the surface of the water source.

Then, the null and alternative hypothesis are:

[tex]H_0: \mu=0\\\\H_a:\mu> 0[/tex]

The significance level is 0.01.

The sample has a size n=10.

The sample mean is M=0.0804.

As the standard deviation of the population is not known, we estimate it with the sample standard deviation, that has a value of s=0.0523.

The estimated standard error of the mean is computed using the formula:

[tex]s_M=\dfrac{s}{\sqrt{n}}=\dfrac{0.0523}{\sqrt{10}}=0.017[/tex]

Then, we can calculate the t-statistic as:

[tex]t=\dfrac{M-\mu}{s/\sqrt{n}}=\dfrac{0.0804-0}{0.017}=\dfrac{0.08}{0.017}=4.861[/tex]

The degrees of freedom for this sample size are:

[tex]df=n-1=10-1=9[/tex]

This test is a right-tailed test, with 9 degrees of freedom and t=4.861, so the P-value for this test is calculated as (using a t-table):

[tex]P-value=P(t>4.861)=0.00045[/tex]

As the P-value (0.00045) is smaller than the significance level (0.01), the effect is  significant.

The null hypothesis is rejected.

There is enough evidence to support the claim that the zinc concentration is greater on the bottom than the surface of the water source.

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