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.

$\pi \pm z\sqrt{\frac{\pi(1-\pi)}{n}}$

In which

z is the zscore that has a pvalue of $1 - \frac{\alpha}{2}$ .

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

This means that $n = 500, \pi = \frac{350}{500} = 0.75$

80% confidence level

So $\alpha = 0.2$ , z is the value of Z that has a pvalue of $1 - \frac{0.2}{2} = 0.9$ , so $Z = 1.28$ .

The lower limit of this interval is:

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

The upper limit of this interval is:

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

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

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3 years ago