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In the transmission of digital information, the probability that a bit has high, moderate, or low distortion is \(0.01,0.04,\) and \(0.95,\) respectively. Suppose that three bits are transmitted and that the amount of distortion of each bit is assumed to be independent. Let \(X\) and \(Y\) denote the number of bits with high and moderate distortion out of the three transmitted, respectively. Determine the following: (a) The probability that two bits have high distortion and one has moderate distortion (b) The probability that all three bits have low distortion (c) The probability distribution, mean, and variance of \(X\) (d) The conditional probability distribution, conditional mean, and conditional variance of \(X\) given that \(Y=2\)

Short Answer

Expert verified
(a) 0.000012, (b) 0.857375, (c) Binomial with mean 0.03, variance 0.0297; (d) Conditional: P(X=0|Y=2)=0.99, E[X|Y=2]=0.01, Var=0.0099.

Step by step solution

01

Understanding the Problem

We have three types of distortion: high (0.01), moderate (0.04), and low (0.95) with respective probabilities for each bit. We need to find different probabilities related to three transmitted bits.
02

Calculate the Probability for High and Moderate Distortion

For part (a), we need exactly 2 bits with high distortion (probability = 0.01 each) and 1 bit with moderate distortion (probability = 0.04). Using the binomial probability formula for high distortion, \( P(X = 2, Y = 1) = \binom{3}{2} (0.01)^2 (0.04)^1 (0.95)^{0} = 3 \times 0.0001 \times 0.04 = 0.000012 \).
03

Calculate the Probability for All Low Distortion

For part (b), we need all three bits with low distortion. Since this is the event where no high or moderate distortion occurs, the probability is simply \( (0.95)^3 = 0.857375 \).
04

Determine Probability Distribution of X

For part (c), since high distortion is a binomial random variable \( X \sim Bin(n=3, p=0.01) \). Therefore, probability function is \( P(X=k) = \binom{3}{k} (0.01)^k (0.99)^{3-k} \) for \( k = 0, 1, 2, 3 \).
05

Calculate Mean and Variance of X

For a binomial distribution \( X \sim Bin(n, p) \), the expected mean is \( E[X] = np = 3 \times 0.01 = 0.03 \) and variance is \( Var(X) = np(1-p) = 3 \times 0.01 \times 0.99 = 0.0297 \).
06

Conditional Probability Distribution Given Y=2

For part (d), if 2 bits have moderate distortion, only 1 bit remains. Since the number of bits that can have high distortion is then between 0 and 1, the conditional probabilities are \( P(X=0|Y=2) = 0.99 \), \( P(X=1|Y=2) = 0.01 \).
07

Conditional Mean and Variance Given Y=2

Given \( Y=2 \), \( X \) can only be 0 or 1, so the conditional mean is \( E[X|Y=2] = 0 \times 0.99 + 1 \times 0.01 = 0.01 \), and the conditional variance is \( E[(X-0.01)^2] = (0-0.01)^2 \times 0.99 + (1-0.01)^2 \times 0.01 = 0.0099 \).
08

Conclusion: Assemble the Solutions

(a) The probability is \( 0.000012 \). (b) The probability is \( 0.857375 \). (c) The distribution is binomial with mean \( 0.03 \) and variance \( 0.0297 \). (d) Conditional distribution is for \( X=0 \) or \( X=1 \) with conditional mean \( 0.01 \) and variance \( 0.0099 \).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Binomial Distribution
The binomial distribution is a fundamental concept in probability, widely used in various engineering applications. It models the number of successes in a fixed number of independent trials of a binary experiment, such as flipping a coin or, as in our exercise, assessing distortion in digital information transmission. Each trial results in one of two outcomes, often termed "success" and "failure," though these terms are simply labels. In the exercise, the probability of high distortion in a transmitted bit was given as 0.01. Here, the number of trials is 3 because we transmit three bits. The exercise sought the number of bits with high distortion, which we solved using the binomial probability formula:\[ P(X=k) = \binom{3}{k} (0.01)^k (0.99)^{3-k} \]In this formula, \(\binom{3}{k}\) represents the number of ways to choose \(k\) high distortion bits out of 3. Understanding this distribution helps engineers design better communication systems.
Conditional Probability
Conditional probability provides a way to update the probability of an event based on the occurrence of another event. In simpler terms, it's about finding the likelihood of an event, given that another event has occurred. It's fundamental in engineering, especially in systems where conditions change and affect outcomes. In the exercise, we were interested in the conditional probability of high distortion given that we have moderate distortion in 2 of the 3 transmitted bits. Conditional probabilities are useful because they allow engineers to focus on specific scenarios and understand how changes in one component can affect the entire system. The specific calculations for conditional probability in this context revolved around understanding if 2 bits are moderately distorted, the probability distribution for high distortion changes because there are fewer bits left to consider for high distortion.
Mean and Variance
Mean and variance are crucial statistical tools in assessing and predicting the behavior of a distribution. The mean, or expected value, gives the average outcome if an experiment is repeated many times, providing a central value around which data points lie. Variance, on the other hand, gives information about the spread of the distribution around the mean.For a binomial distribution as seen in the exercise, the mean is calculated by multiplying the number of trials by the probability of success: \[ E[X] = np \]With our parameters \( n = 3 \) and \( p = 0.01 \), the mean is 0.03. Variance is calculated by \( np(1-p) \), which in our case is 0.0297. This helps engineers predict how much spread to expect around the mean, critical for quality control in digital transmission.
Digital Information Transmission
Digital information transmission involves sending data in binary format over networks. This process can be influenced by various factors leading to distortion, which is detrimental to the quality and reliability of the data received. Engineers strive to understand and reduce distortion to ensure smoother, more accurate transmission. In our exercise, we were looking at different levels of distortion: high, moderate, and low, each with its associated probability. Mitigating these distortions is an ongoing challenge in digital communication, and understanding the probability of different distortion levels is crucial. By addressing the probabilities and distributions of these distortion levels, engineers can design systems with improved error correction and data integrity measures, paving the way for more robust communication channels.

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