Acid rain accumulations in lakes and streams in the northeastern part of the United States are a major environmental concern. A researcher wants to know what fraction of lakes contain hazardous pollution levels. He randomly selects 200 lakes and determines that 45 of the selected lakes have as unsafe concentration of acid rain pollution.

(a) Calculate the best point estimate of the population proportion of lakes that have unsafe concentrations of acid rain pollution.

(b) Determine a 95% confidence interval for the population proportion.

(c) If a local politician states that only 20% of the lakes are contaminated, does the study provide overwhelming evidence at the 95% level to contradict his views?

In a sample with a number n of people surveyed with a probability of a success of [tex]\pi[/tex], and a confidence level of [tex]1-\alpha[/tex], we have the following confidence interval of proportions.

[tex]\pi \pm z\sqrt{\frac{\pi(1-\pi)}{n}}[/tex]

In which

z is the zscore that has a pvalue of [tex]1 - \frac{\alpha}{2}[/tex].

For this problem, we have that:

[tex]n = 200, \pi = 0.225[/tex]

95% confidence level

So [tex]\alpha = 0.05[/tex], z is the value of Z that has a pvalue of [tex]1 - \frac{0.05}{2} = 0.975[/tex], so [tex]Z = 1.96[/tex].

The lower limit of this interval is:

[tex]\pi - z\sqrt{\frac{\pi(1-\pi)}{n}} = 0.225 - 1.96\sqrt{\frac{0.225*0.775}{200}} = 0.167[/tex]

The upper limit of this interval is:

[tex]\pi + z\sqrt{\frac{\pi(1-\pi)}{n}} = 0.225 + 1.96\sqrt{\frac{0.225*0.775}{200}} = 0.283[/tex]

The 95% confidence interval for the population proportion is (0.167, 0.283).

(c) If a local politician states that only 20% of the lakes are contaminated, does the study provide overwhelming evidence at the 95% level to contradict his views?

No, since 20% = 0.2 is part of the confidence interval.