Question

What sample size is needed to give the desired margin of error in estimating a population mean with the indicated level of confidence? A margin of error within ±12 with \(95 \%\) confidence, assuming we estimate that \(\sigma \approx 125\)

Short answer

The required sample size is 267

Step-by-step solution

Identify the parameters

First, identify the given parameters. Here, the margin of error \( E = 12 \), the standard deviation \( \sigma = 125 \), and the confidence level is \( 95 \% \).

Find the Z value

Next, look up the z-value in a standard Z-table which corresponds to the given confidence level. For a \( 95 \% \) confidence level, the Z value is about \( 1.96 \).

Calculating the sample size

Now, plug into the formula \[ n = \left(\frac{Z\sigma}{E}\right)^2 \]. Substitute \( Z = 1.96 \), \( \sigma = 125 \), and \( E = 12 \) into the formula to get \[ n = \left(\frac{1.96 * 125}{12}\right)^2 \]. Calculating this gives approximately \( 267.35 \). Since a sample size must be a whole number, round this value.

Final Answer

Lastly, round off the calculated value to the nearest whole number. In this case, rounding \( 267.35 \) gives \( 267 \). So, a sample size of \( 267 \) would be needed to achieve the desired margin of error with a confidence level of \( 95 \% \).

Key concepts

Margin of Error

Understanding the margin of error is crucial when conducting statistical analysis for population mean estimation. The margin of error represents how much you can expect your survey results to reflect the views of the actual population. In simpler terms, it gives us an interval where we expect the true population parameter to fall. For instance, in the provided exercise, a margin of error of ±12 means that the estimated population mean is likely to be within 12 units (either above or below) of the sample mean.

As a general rule, a smaller margin of error requires a larger sample size to ensure the true population mean lies within that specified range. It's important because it reflects the precision of the estimate; a lower margin of error indicates a more precise estimate, which is often desired in research.

Confidence Level

The confidence level is a measure of certainty regarding how well a sample represents the population. A confidence level of 95%, as used in our problem, indicates that if 100 different samples were taken and interval estimates were made for each sample, we would expect the population mean to fall within those intervals in 95 out of 100 cases. The confidence level is denoted by the symbol \(1 - \alpha\), where \(\alpha\) is the probability of making a Type I error (rejecting a true null hypothesis).

The choice of confidence level affects the Z-value used for calculating the sample size. Higher confidence levels correspond to larger Z-values, which result in larger sample sizes needed to maintain the same margin of error.

Standard Deviation

Standard deviation (\(\sigma\)) is a key component in determining the necessary sample size for accurate population mean estimations. It quantifies the amount of variation or dispersion in a set of values. A low standard deviation means that the values tend to be close to the mean (average) of the set, while a high standard deviation means that the values are spread out over a wider range.

In the context of sample size calculation, a larger standard deviation suggests more variability in the population and typically necessitates a larger sample size to get an accurate estimate within the desired margin of error. In the example provided, the standard deviation is assumed to be \(125\), which influences the sample size needed to achieve the specified accuracy.

Z-value

The Z-value, also known as the Z-score, is a critical number that reflects how many standard deviations an element is from the mean. In the scenario of confidence intervals, the Z-value corresponds to the desired confidence level. It's obtained from the standard normal distribution table which indicates the probability of a score falling within a particular range.

For a confidence level of 95%, the Z-value is approximately 1.96. This Z-value is used in the sample size formula to ensure that the resulting confidence interval has the correct properties. The higher the Z-value, the larger the sample size needed because it widens the confidence interval, thereby decreasing the likelihood that the interval will not contain the population mean.

Population Mean Estimation

Population mean estimation involves predicting the average value of a characteristic within an entire population based on a sample drawn from that population. The goal is to estimate the mean with a certain degree of precision and confidence. The exercise in question demonstrates how to determine the sample size necessary to estimate the population mean with a given margin of error and confidence level.

To achieve this, we use the sample size formula which incorporates the margin of error, standard deviation, and Z-value appropriate to the confidence level. These factors together will determine the minimum number of observations required to ensure that the estimated population mean is sufficiently close to the true population mean.