Question

We examine the effect of different inputs on determining the sample size needed to obtain a specific margin of error when finding a confidence interval for a proportion. Find the sample size needed to give a margin of error to estimate a proportion within \(\pm 3 \%\) with \(99 \%\) confidence. With \(95 \%\) confidence. With \(90 \%\) confidence. (Assume no prior knowledge about the population proportion \(p\).) Comment on the relationship between the sample size and the confidence level desired.

Short answer

The required sample sizes for 99%, 95% and 90% confidence intervals are 1860, 1068, and 751 respectively. The larger the desired confidence level, the larger the required sample size.

Step-by-step solution

Calculate the Z-Score

The first step is to calculate the Z-score for each confidence level. The Z-Scores for 99% confidence, 95% confidence and 90% confidence are approximately 2.58, 1.96 and 1.645 respectively.

Apply the Margin of error formula

We rearrange the error bound formula: \(E = z * \sqrt{\frac{p(1-p)}{n}}\) to be in the form \(n = (\frac{z}{E})^2 * p(1-p)\) and use it to calculate the sample size \(n\), applying the worst case scenario \(p=0.5\) for each confidence level. For 99% confidence interval, \(n =(\frac{2.58}{0.03})^2 * 0.5*0.5 = 1860\). Similarly for 95% confidence interval, \(n = (\frac{1.96}{0.03})^2 * 0.5*0.5 = 1068\), and for 90% confidence interval, \(n =(\frac{1.645}{0.03})^2 * 0.5*0.5 = 751\).

Round up to the nearest whole number

Sample size should always be a whole number. So, if you got a decimal in step 2, round up to the nearest whole number because you can't really have a fraction of a sample. Therefore, the sample sizes are 1860 for 99%, 1068 for 95% and 751 for 90%.

Analysis of the relationship between sample size and confidence level

With all else being equal, the higher the desired confidence level, the larger the required sample size. This is because increasing the confidence level requires a wider confidence interval, which requires a larger sample size.

Key concepts

Confidence Interval

When statisticians estimate the characteristics of a population, like the average height or the proportion of people favoring a certain policy, they use a statistic known as a confidence interval (CI). This interval gives a range within which we can say, with a specified probability, that the true population parameter lies. The most common confidence levels are 90%, 95%, and 99%.

For example, a 95% CI for the population proportion might be expressed as [0.45, 0.55]. This means we are 95% confident that the true proportion is between 45% and 55%. However, exactly 95% confidence doesn't mean the parameter will definitely fall within the range; there's still a 5% chance it won't. This possible error is where the concept of margin of error comes into play.

Imagine you're trying to estimate the usage of a new educational app among college students. If your 95% CI is [0.20, 0.40], then you can be quite confident that the true proportion of college students using the app is between 20% and 40%. The confidence level and the CI are crucial for researchers and analysts since they provide both a measure and an indicator of the reliability of an estimate.

Margin of Error

The margin of error (ME) is a statistic that expresses the amount of random sampling error in a survey's results. It represents how much the responses in the sample can deviate from the true population value and is directly connected to the confidence interval.

Using the formula for margin of error, which is usually expressed as ME = z * \(\sqrt{\frac{p(1-p)}{n}}\), you can see it depends on three main factors: the Z-score, the population proportion (p), and the sample size (n). In the context of our textbook problem, with an ME of \(\pm 3\%\), we determine how big of a sample we need to be certain about our estimate within that \(\pm 3\%\) range.

Returning to our educational app estimation, if we claim that 30% of college students use the app with a margin of error of \(\pm 5\%\), it means that the true proportion could be as low as 25% or as high as 35%. A smaller margin of error requires a larger sample size, but it provides a closer estimate of the population parameter, which can be very significant in studies that require precise measurements.

Z-score

A Z-score, also known as a standard score, is a statistical measurement that describes a value's relationship to the mean of a group of values. It's measured in terms of standard deviations from the mean. In the context of confidence intervals, the Z-score determines how far out from the mean you have to go to capture the desired proportion of the data.

The higher the Z-score, the further out from the mean, and the wider the confidence interval. This is why, as seen in the textbook solution, different confidence levels have different Z-scores. For a 99% confidence level, we use a Z-score of 2.58 because it reflects that only 1% of the data lies outside the 99% confidence interval, thus requiring a larger sample size to ensure that the estimated parameter falls within this strict criterion.

Understanding Z-scores is key if you're working with normal distributions. Since many statistical methods assume data is normally distributed or can be approximated as such, Z-scores and the standard normal distribution are foundational concepts in statistics. For students grappling with sample size determinations, knowing how the confidence level alters the Z-score, and in turn the required sample size, is crucial for designing reliable studies and experiments.