# Lecture 29: Law of Large Numbers and Central Limit Theorem | Statistics 110 | Summary and Q&A

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April 29, 2013
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Harvard University
Lecture 29: Law of Large Numbers and Central Limit Theorem | Statistics 110

## TL;DR

The Law of Large Numbers states that the sample mean converges to the true mean with a probability of 1, while the Central Limit Theorem states that the distribution of the sample mean approaches a normal distribution as the sample size increases.

## Key Insights

• 🇨🇫 The Law of Large Numbers and the Central Limit Theorem are fundamental theorems in probability theory.
• ❓ The Law of Large Numbers states that the sample mean converges to the true mean with a probability of 1.
• 🇨🇫 The Central Limit Theorem states that, under certain conditions, the distribution of the sample mean approaches a normal distribution as the sample size increases.
• ❓ Both theorems are widely used in statistics to estimate population parameters and make statistical inferences.
• 👻 The Central Limit Theorem allows for normal approximations of the sample mean, which simplifies calculations and provides useful approximations.
• ⚾ The Law of Large Numbers and the Central Limit Theorem are crucial in order for science to be possible, as they provide a framework for estimating population parameters based on sample data.

## Transcript

All right, so let's get started. So today, we're gonna talk about what are probably the two most famous theorems in the entire history of probably. They're called the law of large numbers and the central limit theorem. They're closely related, so makes sense to do them together, kind of compare and contrast them. I don't, I can't think of a more fa... Read More

## Questions & Answers

### Q: What are the two most famous theorems in the history of probability?

The two most famous theorems in the history of probability are the Law of Large Numbers and the Central Limit Theorem.

### Q: What does the Law of Large Numbers state?

The Law of Large Numbers states that the sample mean converges to the true mean with a probability of 1.

### Q: What does the Central Limit Theorem state?

The Central Limit Theorem states that, under certain conditions, the distribution of the sample mean approaches a normal distribution as the sample size increases.

### Q: How can the Law of Large Numbers be applied in practice?

The Law of Large Numbers is used to estimate the true mean of a population by taking the average of a large sample size. It provides a justification for using the sample mean as an approximation for the true mean.

### Q: What is the significance of the Central Limit Theorem in statistics?

The Central Limit Theorem allows for the use of normal approximations when studying the sample mean. This is particularly useful when dealing with large sample sizes, as it simplifies calculations and provides a good approximation.

## Summary

This video discusses two famous probability theorems: the Law of Large Numbers and the Central Limit Theorem. The Law of Large Numbers states that the sample mean converges to the true mean with probability 1 as the sample size increases. The Central Limit Theorem states that as the sample size gets large, the standardized sample mean (subtracted by the true mean and divided by the standard deviation) converges in distribution to the standard normal distribution.

## Questions & Answers

### Q: What is the Law of Large Numbers and what does it state?

The Law of Large Numbers states that as the sample size increases, the sample mean will converge to the true mean with probability 1.

### Q: How do we interpret and use the Law of Large Numbers in practice?

The Law of Large Numbers allows us to estimate the true mean of a population by taking the average of a large sample size. It gives us confidence that the sample mean will be close to the true mean.

### Q: What is the Central Limit Theorem and what does it state?

The Central Limit Theorem states that as the sample size gets large, the standardized sample mean (subtracted by the true mean and divided by the standard deviation) converges in distribution to the standard normal distribution.

### Q: What does it mean for the standardized sample mean to converge in distribution to the standard normal distribution?

Convergence in distribution means that if we take the CDF (cumulative distribution function) of the standardized sample mean, it will converge to the CDF of the standard normal distribution. This implies that the distribution of the standardized sample mean becomes more and more similar to the standard normal distribution as the sample size increases.

### Q: How do we use the Central Limit Theorem to approximate probabilities?

To approximate probabilities, we can standardize the random variable of interest and use the CDF of the standard normal distribution. We subtract the true mean from the random variable, divide it by the standard deviation, and then calculate the probabilities using the standard normal distribution.

### Q: Is the Central Limit Theorem applicable to any type of distribution?

The Central Limit Theorem is applicable to any distribution, as long as the sample size is large enough and some assumptions about the mean and variance are met. However, for the best approximation results, it is ideal to work with distributions that are closer to the normal distribution, particularly with a mean close to one-half.

### Q: How does the Law of Large Numbers relate to the Central Limit Theorem?

The Law of Large Numbers is a simpler version of the Central Limit Theorem. It specifically focuses on the convergence of the sample mean to the true mean as the sample size increases. The Central Limit Theorem, on the other hand, provides more details about the distribution of the sample mean.

### Q: Can you explain the concept of convergence in terms of random variables and distributions?

Convergence of random variables refers to the behavior of a sequence of random variables as some parameter (such as the sample size) changes. Convergence in distribution means that the distribution of a sequence of random variables approaches a specific type of distribution (such as the standard normal distribution) as the parameter changes.

### Q: What assumptions are necessary for the Central Limit Theorem to hold?

The Central Limit Theorem requires the random variables to be independent and identically distributed (i.i.d). It also assumes that the mean and variance of the random variables exist and are finite. These assumptions ensure that the sample mean can be properly standardized and that the distribution of the random variables can converge to the standard normal distribution.

### Q: How does the Law of Large Numbers and the Central Limit Theorem impact statistical estimation and the study of probabilities?

The Law of Large Numbers allows us to estimate population means and other parameters by taking larger sample sizes. It provides confidence that our estimates will be close to the true values. The Central Limit Theorem allows for approximations of probabilities using the normal distribution, making it easier to perform calculations and make inferences based on large sample sizes.

## Takeaways

The Law of Large Numbers and the Central Limit Theorem are two essential theorems in probability theory. The Law of Large Numbers states that the sample mean converges to the true mean with probability 1 as the sample size increases. This theorem is important for statistical estimation and provides a basis for understanding how sample means behave. The Central Limit Theorem, on the other hand, explains that as the sample size becomes large, the standardized sample mean converges in distribution to the standard normal distribution. This theorem allows for the approximation of probabilities using the normal distribution, making calculations and inferences easier. While the Law of Large Numbers gives us confidence in estimating the true mean, the Central Limit Theorem provides a more detailed understanding of the distribution of sample means. Together, these theorems are crucial for the study of probabilities and statistical analysis.

## Summary & Key Takeaways

• The Law of Large Numbers and the Central Limit Theorem are two famous theorems in probability theory that explain the behavior of the sample mean as the sample size increases.

• The Law of Large Numbers states that the sample mean converges to the true mean with a probability of 1.

• The Central Limit Theorem states that, under certain conditions, the distribution of the sample mean approaches a normal distribution as the sample size increases.

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