Expected Value and Standard Deviation

Oct 13, 2025 by wbrum.org 125 views

Introduction

In statistics, sampling distributions play a crucial role in understanding the behavior of sample statistics. The central limit theorem (CLT) is a fundamental concept that describes the distribution of sample means. In this article, we will explore the concept of sampling distributions and the CLT, and apply it to a real-world scenario involving the sale of four pairs of shoes.

Sampling Distributions

A sampling distribution is a probability distribution of a sample statistic, such as the mean or proportion. It is created by taking multiple samples from a population and calculating the sample statistic for each sample. The sampling distribution of a sample statistic is a theoretical distribution that describes the possible values of the sample statistic and their probabilities.

The Central Limit Theorem

The central limit theorem states that the sampling distribution of the sample mean will be approximately normally distributed, regardless of the shape of the population distribution, if the sample size is sufficiently large. This means that the sample mean will be close to the population mean, and the standard deviation of the sample mean will be close to the population standard deviation divided by the square root of the sample size.

Four Pairs of Shoes

Let's consider a scenario where four pairs of shoes are for sale: black boots (B) cost $\$70$ , red boots (R) cost $\$60$ , white sneakers (W) cost $\$50$ , and green sandals (G) cost $\$30$ . We want to find the expected value and standard deviation of the sample mean of the prices of these shoes.

Expected Value and Standard Deviation

To find the expected value and standard deviation of the sample mean, we need to calculate the mean and standard deviation of the population. The population mean is the sum of the prices of the four pairs of shoes divided by 4:

\mu = \frac{70 + 60 + 50 + 30}{4} = 55

The population standard deviation is the square root of the variance of the population. The variance of the population is the sum of the squared differences between each price and the population mean, divided by 4:

\sigma^2 = \frac{(70-55)^2 + (60-55)^2 + (50-55)^2 + (30-55)^2}{4} = 125

The population standard deviation is the square root of the variance:

\sigma = \sqrt{125} = 11.18

Sampling Distribution of the Sample Mean

The sampling distribution of the sample mean is approximately normally distributed, with a mean of 55 and a standard deviation of 11.18 divided by the square root of the sample size. Let's consider a sample size of 4. The standard deviation of the sample mean is:

\sigma_{\bar{x}} = \frac{\sigma}{\sqrt{n}} = \frac{11.18}{\sqrt{4}} = 5.59

Identifying Samples of Size 4

We want to identify all samples of size 4 from the four pairs of shoes. A sample of size 4 can be represented as a combination of 4 items from the 4 pairs of shoes. The possible samples are:

B, B, B, B
B, B, B, R
B, B, B, W
B, B, B, G
B, B, R, R
B, B, R, W
B, B, R, G
B, B, W, W
B, B, W, G
B, B, G, G
B, R, R, R
B, R, R, W
B, R, R, G
B, R, W, W
B, R, W, G
B, R, G, G
B, W, W, W
B, W, W, G
B, W, G, G
B, G, G, G
R, R, R, R
R, R, R, W
R, R, R, G
R, R, W, W
R, R, W, G
R, R, G, G
R, W, W, W
R, W, W, G
R, W, G, G
R, G, G, G
W, W, W, W
W, W, W, G
W, W, G, G
W, G, G, G
G, G, G, G

Conclusion

In this article, we explored the concept of sampling distributions and the central limit theorem. We applied these concepts to a real-world scenario involving the sale of four pairs of shoes. We calculated the expected value and standard deviation of the sample mean of the prices of these shoes, and identified all samples of size 4 from the four pairs of shoes. The sampling distribution of the sample mean is approximately normally distributed, with a mean of 55 and a standard deviation of 5.59.

Q: What is a sampling distribution?

A: A sampling distribution is a probability distribution of a sample statistic, such as the mean or proportion. It is created by taking multiple samples from a population and calculating the sample statistic for each sample.

Q: What is the central limit theorem?

A: The central limit theorem states that the sampling distribution of the sample mean will be approximately normally distributed, regardless of the shape of the population distribution, if the sample size is sufficiently large.

Q: What is the expected value of a sampling distribution?

A: The expected value of a sampling distribution is the population mean. It is the value that the sample statistic is likely to take on in the long run.

Q: What is the standard deviation of a sampling distribution?

A: The standard deviation of a sampling distribution is the population standard deviation divided by the square root of the sample size. It measures the amount of variation in the sample statistic.

Q: What is the relationship between the sample size and the standard deviation of the sampling distribution?

A: The standard deviation of the sampling distribution decreases as the sample size increases. This is because the sample size is in the denominator of the formula for the standard deviation.

Q: Can the central limit theorem be applied to proportions?

A: Yes, the central limit theorem can be applied to proportions. The sampling distribution of the sample proportion will be approximately normally distributed, regardless of the shape of the population distribution, if the sample size is sufficiently large.

Q: What is the importance of the central limit theorem?

A: The central limit theorem is important because it allows us to make inferences about a population based on a sample of data. It provides a way to estimate the population mean and standard deviation, and to make predictions about the behavior of the sample statistic.

Q: Can the central limit theorem be applied to categorical data?

A: No, the central limit theorem cannot be applied to categorical data. The central limit theorem is typically applied to continuous data, such as the prices of shoes in the example above.

Q: What is the difference between the sampling distribution and the population distribution?

A: The sampling distribution is a probability distribution of a sample statistic, while the population distribution is the probability distribution of the population. The sampling distribution is a theoretical distribution that describes the possible values of the sample statistic, while the population distribution is the actual distribution of the population.

Q: Can the central limit theorem be applied to small sample sizes?

A: No, the central limit theorem typically requires a sufficiently large sample size to be applicable. The sample size must be large enough to ensure that the sampling distribution of the sample mean is approximately normally distributed.

Q: What is the relationship between the sample size and the accuracy of the central limit theorem?

A: The accuracy of the central limit theorem increases as the sample size increases. This is because the sample size is in the denominator of the formula for the standard deviation, and a larger sample size results in a smaller standard deviation.

Q: Can the central limit theorem be applied to non-normal data?

A: Yes, the central limit theorem can be applied to non-normal data. The sampling distribution of the sample mean will be approximately normally distributed, regardless of the shape of the population distribution, if the sample size is sufficiently large.

Q: What is the importance of understanding the central limit theorem?

A: Understanding the central limit theorem is important because it provides a way to make inferences about a population based on a sample of data. It allows us to estimate the population mean and standard deviation, and to make predictions about the behavior of the sample statistic.