We calculated the last time that the largest allowed arsenic concentration was 1

ID: 3340292 • Letter: W

Question

We calculated the last time that the largest allowed arsenic concentration was 10 units per billion. The measurement at one home were

2,4,6,6,8. Question is: Is it safe to drink water?

We first calculated the 95% confidence interval.

We are interested only in the upper limit of that interval. The upper limit is (do not round just write down the first two number of the result)

According to this the water is (choose one of the two option)

(a) safe to drink

(b) not safe to drink

What if we want to be 99% confident? What would be the upper limit of 99% confident interval?

The upper limit is (do not round just write down the first two number of the result)

According to this we are 99% confident that water is (choose one of the two option)

(a) safe to drink

(b) not safe to drink

Ans:

From sampled data:

n=5

sample mean=5.2

std dev=2.28

t value for 95% CI and df=5-1=4 is 2.78

95% confidence interval

=5.2+/-2.78*(2.28/sqrt(5))

=5.2+/-2.83

=(2.37,8.03)

Upper limit=8.03

As,upper CI limits is below 10,so it is safe to drink.(Option a is correct)

t value for 99% CI and df=5-1=4 is 4.60

Now,99% CI

=5.2+/-4.60*(2.28/sqrt(5))

=5.2+4.70

=(0.50,9.90)

Upper limit=9.90

As,iupper limit is below10,it is safe to drink.(Option a is correct)

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