Problem 28 - Algebra 2

Question

An event occurs \(x\) times in a sample of size \(n .\) Find its sample proportion and margin of error. $$ \begin{array}{l}{x=100} \\ {n=250}\end{array} $$

Step-by-Step Solution

Answer

The sample proportion is 0.4 and the margin of error is 0.09604.

Step 1: Calculate the sample proportion

The sample proportion \(p\) is given by the formula \(p = x / n\). Plugging the given numbers into the formula gives \(p = 100 / 250 = 0.4\).

Step 2: Calculate the standard deviation of the proportion

The standard deviation of the proportion \(\sigma\) is given by the formula \(\sigma = \sqrt{p(1 - p) / n}\), where \(p\) is the sample proportion, \(n\) is the sample size, and \((1 - p)\) is the complement of the proportion. Plugging the given numbers into the formula gives \(\sigma = \sqrt{0.4(1 - 0.4) / 250} = \sqrt{0.6 / 250} = \sqrt{0.0024} =0.049\).

Step 3: Calculate the margin of error

The margin of error \(E\) is given by the formula \(E = z* \sigma\), where \(z*\) is the z-score, which we take to be 1.96 for a 95% confidence level, and \(\sigma\) is the standard deviation of the proportion. Substituting the given numbers into the formula gives \(E = 1.96 * 0.049 = 0.09604\).

Key Concepts

Margin of Error

The margin of error is a crucial aspect of statistics, especially when dealing with estimates from sample data. It tells us how much we can expect the results of our sample to vary from the true population parameter. In simple terms, the margin of error provides a range that our true population parameter is likely to fall within. For example, if you were to say that 40% of a town's population likes a certain type of music, the margin of error will tell you how precise this 40% estimate is when translated to the entire population.

To calculate the margin of error, you'll usually multiply the standard deviation by a z-score, which corresponds to your confidence level. A z-score of 1.96 is commonly used for a 95% confidence level, as it indicates that the true parameter should fall within this range 95% of the time when repeated samples are taken. In the exercise, we calculated the margin of error to be approximately 0.09604, meaning our true population proportion could be as low as 31% or as high as 49%.

Confidence Interval

A confidence interval is a range around a sample statistic that estimates its accuracy and reliability. It tells you how confident you can be about the sample statistic in reflecting the true population parameter. It's not just a single number; instead, it's an interval that is meant to capture the true parameter a specified percentage of the time, often 95% or 99% of the time.

The confidence interval for a proportion can be found using the formula: \[ p \pm E \]where \( p \) is the sample proportion and \( E \) is the margin of error. From our exercise's example, where the sample proportion \( p \) is 0.4 and the margin of error \( E \) is approximately 0.09604, the confidence interval would be:

• Lower bound: 0.4 - 0.09604 = 0.30396
• Upper bound: 0.4 + 0.09604 = 0.49604

So, we are 95% confident that the true population proportion falls between 30.4% and 49.6%.

Standard Deviation of Proportion

The standard deviation of a proportion gives you an idea of how much variation or dispersion exists in your sample proportions. When you're estimating population parameters based on a sample, this standard deviation helps tell you how 'spread out' your proportion estimates might be from sample to sample.

The formula for the standard deviation of a proportion is:\[ \sigma = \sqrt{\frac{p(1-p)}{n}} \]where \( p \) is the sample proportion, \( 1 - p \) is the complement of the proportion, and \( n \) is the sample size. This formula takes into account the sample size, so the larger your sample, the smaller your standard deviation will tend to be, which means more precise estimates.

In the given exercise, when we calculated \( \sigma \) for \( p = 0.4 \) and \( n = 250 \), we found it to be 0.049. This value tells us that our sample proportion of 0.4 could reasonably vary by about 4.9% from sample to sample, due to random sampling error.