Question

The results of a Newsweek poll concerning views on abortion given in Exercise 7.66 showed that of \(n=1002\) adults, \(39 \%\) favored the "right-to-life" stand, while \(53 \%\) were "pro-choice." The poll reported a margin of error of plus or minus \(3 \%\). a. Construct a \(90 \%\) confidence interval for the proportion of adult Americans who favor the "right-tolife" position. b. Construct a \(90 \%\) confidence interval for the proportion of adult Americans who favor the "pro-choice" position.

Short answer

Answer: The 90% confidence interval for the Right-to-Life position is between 35.95% and 42.05%, and the 90% confidence interval for the Pro-Choice position is between 49.9% and 56.1%.

Step-by-step solution

Find the sample proportion and critical value for Right-to-Life position

First, we need to determine the sample proportion for the Right-to-Life position, which is 39%. Therefore, \(\hat{p} = 0.39\). We also need the 90% critical value (z-value), which we can look up or calculate as \(z_\frac{\alpha}{2} = 1.645\).

Calculate the Right-to-Life Confidence Interval

Now we can use the formula for the confidence interval: \(CI = 0.39 \pm 1.645 \times \sqrt{\frac{0.39(1-0.39)}{1002}}\) Calculate the margin of error: \(1.645 \times \sqrt{\frac{0.39(1-0.39)}{1002}} = 0.0305\) So, the confidence interval for the Right-to-Life position is: \(0.39 \pm 0.0305 = (0.3595, 0.4205)\) This means that we are 90% confident that the true proportion of adult Americans who favor the Right-to-Life position is between 35.95% and 42.05%. #b. Confidence Interval for Pro-Choice Position#

Find the sample proportion and critical value for Pro-Choice position

Similar to part a, we need to determine the sample proportion and critical value for the Pro-Choice position. We know that \(\hat{p} = 0.53\) for this position and, again, we have \(z_\frac{\alpha}{2} = 1.645\).

Calculate the Pro-Choice Confidence Interval

Using the formula for the confidence interval: \(CI = 0.53 \pm 1.645 \times \sqrt{\frac{0.53(1-0.53)}{1002}}\) Calculate the margin of error: \(1.645 \times \sqrt{\frac{0.53(1-0.53)}{1002}} = 0.0310\) So, the confidence interval for the Pro-Choice position is: \(0.53 \pm 0.0310 = (0.499, 0.561)\) This means that we are 90% confident that the true proportion of adult Americans who favor the Pro-Choice position is between 49.9% and 56.1%.

Key concepts

Sample Proportion

When conducting a survey or poll, researchers aim to understand the opinions or characteristics of a larger population by examining a segment of that population, called a sample. The sample proportion is a key statistic that represents the fraction of the sample with a certain characteristic or opinion.

In the context of the given exercise, researchers used a sample of 1002 adults to gauge the public's stance on abortion. They determined that 39% of the sample favored the 'right-to-life' position, meaning the sample proportion, denoted by \(\hat{p}\), is 0.39. Similarly, for the 'pro-choice' stance, the sample proportion is 53%, or \(\hat{p} = 0.53\). Understanding sample proportion is crucial, as it is the starting point for calculating the confidence interval, an estimate that aims to reflect the opinion of the overall population.

Critical Value

When creating a confidence interval, the critical value plays a crucial role. It determines the range within which the true population parameter is expected to lie with a specific level of confidence. The critical value is derived from the probability distribution of the statistic — often a z-distribution for large sample sizes when the population standard deviation is known or can be estimated.

For a given confidence level, say 90%, we want to find the z-value that corresponds to the middle 90% of the z-distribution. The remaining 10% is split between the two tails of the distribution (5% in each tail if looking for a two-tailed interval). In our exercise, this value is \(z_\frac{\alpha}{2} = 1.645\). This critical value helps ensure that if we were to repeat our sampling process numerous times, 90% of the confidence intervals we calculate would contain the true population proportion. Hence, understanding and finding the correct critical value is fundamental in constructing accurate confidence intervals.

Margin of Error

The margin of error is a statistic that indicates the likely size of the error in a sample estimate when compared to the actual value in the full population. It's an essential part of any confidence interval calculation, as it sets the bounds for the range of values within which the population parameter is expected to lie. In layman's terms, it tells us how much 'wiggle room' we should expect in our estimate due to the fact that we're using a sample and not surveying the entire population.

For example, in the exercise regarding the 'right-to-life' position, the margin of error was calculated to be \(1.645 \times \sqrt{\frac{0.39(1-0.39)}{1002}} = 0.0305\). This value is then used to determine the confidence interval, resulting in a range of 35.95% to 42.05%. It reflects our uncertainty inherent in the estimation process, and taking it into account helps us construct a realistic interval that acknowledges the limitations of our sample. The margin of error is influenced by factors such as sample size, variability in the data, and the desired confidence level.