# L13.8 A Simple Example | Summary and Q&A

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April 24, 2018
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MIT OpenCourseWare
L13.8 A Simple Example

## TL;DR

This analysis explores the concept of conditional expectation and conditional variance, using a random variable X described by a PDF with different ranges. It explains how to calculate the conditional expectation and variance, as well as the expected value and total variance.

## Questions & Answers

### Q: What is the purpose of dividing and conquering when dealing with a PDF consisting of different pieces?

Dividing and conquering allows us to consider two different scenarios based on the range of X. By doing this, we can calculate the conditional expectation and variance separately for each scenario, leading to a more accurate probabilistic description.

### Q: How is the conditional expectation of X calculated when Y falls in a specific range?

If Y falls within a certain range, the conditional expectation of X is equal to the midpoint of that interval. For example, if Y is equal to 1, the conditional expectation of X is 1/2.

### Q: What is the probability of Y taking a value of 1?

The probability that Y is equal to 1 can be calculated by finding the area under the PDF in the corresponding range. In this case, since the height of the PDF is 1/2, the probability is also 1/2.

### Q: How is the conditional variance of X calculated when Y takes a certain value?

If Y is equal to 1, the conditional variance of X is 1/12, as X has a uniform PDF on an interval of length one. If Y is equal to 2, the conditional variance of X is 4/12, as X is uniform on an interval of length 2.

### Q: How is the expected value of the conditional variance calculated?

The expected value of the conditional variance can be found by considering the probabilities of each scenario occurring. In this case, with probability 1/2, the conditional variance is 1/12, and with probability 1/2, it is 4/12. The expected value is then calculated accordingly.

### Q: How is the variance of the conditional expectation calculated?

The variance of the conditional expectation is calculated by subtracting the mean of the conditional expectation from each value of the random variable and squaring the result. These terms are then weighted by their respective probabilities.

### Q: What is the total variance and how is it calculated?

The total variance is the sum of the expected value of the variance and the variance of the expected value. In this case, the total variance evaluates to 37/48 when the calculations are carried out.

## Summary & Key Takeaways

• The analysis discusses the concept of conditional expectation and variance in the context of a random variable X described by a PDF with different ranges.

• It explains how to calculate the conditional expectation of X given a certain scenario and the probability of that scenario occurring.

• The analysis also discusses the conditional variance of X based on different scenarios and how to calculate its expected value.

• Finally, it explores the variance of the conditional expectation and the total variance using the law of total variance.