Question

There are 9,481 eligible voters in a precinct. 500 were selected at random and asked to indicate whether they planned to vote for the Democratic incumbent or the Republican challenger. Of the 500 surveyed, 350 said they were going to vote for the Democratic incumbent. Using an 80% confidence level, what are the confidence limits for the proportion that plan to vote for the Democratic incumbent

Answer 1

The confidence limits for the proportion that plan to vote for the Democratic incumbent are 0.725 and 0.775.

Explanation:

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

`\pi \pm z\sqrt{(\pi(1-\pi))/(n)}`

In which

z is the zscore that has a pvalue of
`1 - (\alpha)/(2)`.

Of the 500 surveyed, 350 said they were going to vote for the Democratic incumbent.

This means that
`n = 500, \pi = (350)/(500) = 0.75`

80% confidence level

So
`\alpha = 0.2`, z is the value of Z that has a pvalue of
`1 - (0.2)/(2) = 0.9`, so
`Z = 1.28`.

The lower limit of this interval is:

`\pi - z\sqrt{(\pi(1-\pi))/(n)} = 0.75 - 1.28\sqrt{(0.75*0.25)/(500)} = 0.725`

The upper limit of this interval is:

`\pi + z\sqrt{(\pi(1-\pi))/(n)} = 0.75 + 1.28\sqrt{(0.75*0.25)/(500)} = 0.775`

The confidence limits for the proportion that plan to vote for the Democratic incumbent are 0.725 and 0.775.