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A system has a random number of flaws that we will suppose is Poisson distributed with mean \(c\). Each of these flaws will, independently, cause the system to fail at a random time having distribution \(G\). When a system failure occurs, suppose that the flaw causing the failure is immediately located and fixed. (a) What is the distribution of the number of failures by time \(t\) ? (b) What is the distribution of the number of flaws that remain in the system at time \(t\) ? (c) Are the random variables in parts (a) and (b) dependent or independent?

Short Answer

Expert verified
The number of failures by time t follows a compound Poisson distribution with parameters c and G. The number of remaining flaws at time t follows a binomial distribution with parameters c and \(P(T > t)\). The random variables in parts (a) and (b) are dependent since the remaining flaws in the system directly affect the number of failures.

Step by step solution

01

(a) Distribution of the Number of Failures by Time t

: We are given that the number of flaws in the system follows a Poisson distribution with mean c. And each of these flaws has a distribution G as the system failure time. Since each flaw independently causes the system to fail, we can find the probability that the system will fail because of one flaw up to time t. Let's denote this probability as \(P(T \leq t)\) representing the probability that system failure will occur by time t. Given a Poisson distribution, we need to find the probability that the failure occurs n times in the time interval [0,t]. This is represented as \(P(N(t) = n)\), where \(N(t)\) stands for the number of failures by time t. This can be modeled by a compound Poisson distribution. Therefore, \(P(N(t) = n) = \sum_{k=1}^{\infty} e^{-c} \frac{c^k}{k!} { k \choose n} [P(T \leq t)]^n [1 - P(T \leq t)]^{k-n} \) This shows that the number of failures by time t follows a compound Poisson distribution with parameters c and G.
02

(b) Distribution of the Number of Remaining Flaws at Time t

: To find the distribution of the number of remaining flaws at time t, we can start by finding the probability that a flaw still remains at time t. Since each flaw independently causes the failure, the probability of a flaw still remaining in the system is the complement of the probability that the system will fail because of this flaw up to time t. Let's denote this probability as \(P(T > t)\), which represents the probability that the system will not fail because of one particular flaw by time t. Now, let's denote the number of remaining flaws at time t as X(t). Since there are initially c flaws in the system that are Poisson distributed, to find the distribution of the number of remaining flaws, we can use the binomial theorem: \(P(X(t) = k) = {c \choose k} [P(T > t)]^k [1 - P(T > t)]^{c-k}\) This shows that the number of remaining flaws at time t follows a binomial distribution with parameters c and \(P(T > t)\).
03

(c) Determining Dependency or Independence of Random Variables

: Now we need to check whether the random variables in parts (a) and (b) are dependent or independent. We have shown that the number of failures at time t follows a compound Poisson distribution, and the number of remaining flaws at time t follows a binomial distribution. We can find the joint probability of the number of failures at time t and the number of remaining flaws at time t, denoted as \(P(N(t) = n, X(t) = k)\). If this joint probability is equal to the product of the individual probabilities, i.e., \(P(N(t) = n, X(t) = k) = P(N(t) = n) * P(X(t) = k)\), then the random variables are independent. Otherwise, they are dependent. For this problem, since the remaining flaws in the system directly affect the number of failures, we can conclude that the number of failures and the number of remaining flaws are dependent random variables.

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Key Concepts

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

Compound Poisson Distribution
A compound Poisson distribution is a statistical model that combines the characteristics of both a Poisson distribution, which counts occurrences over a fixed period or in a fixed space, and another distribution which provides the distribution of the magnitudes of the occurrences.

In our system failure context, the compound Poisson distribution describes the number of failures by time t considering each flaw can cause independent failures with a distribution G. When we depict the system failure events, the compound aspect occurs because the number of flaws itself is random, following a Poisson distribution, and the individual failures also hold their own random pattern modeled by G. Effectively, a compound Poisson encompasses both the randomness of the number of flaws and the randomness of the failures they cause.

The compound Poisson distribution is crucial in reliability analysis, risk assessment, and insurance claim modeling, where the events and their impacts are inherently random and interlinked.
System Failure Probability
The system failure probability is a vital metric in reliability engineering, capturing the likelihood that a system will fail within a certain timeframe. In our scenario, each flaw in the system has a chance to cause a failure independently, following a distribution G. By computing the probability that no flaw will cause the system to fail by time t or alternatively, that at least one flaw will lead to failure, we can assess the reliability of the system.

To improve understanding of this concept in educational content, it can be useful to use practical examples and visualize the timeline of potential failures. Running simulations with different means (c) and failure time distributions (G) could also help students better grasp the concept of system failure probability.
Binomial Distribution
The binomial distribution is a foundational concept in statistics that deals with the number of successes in a sequence of independent experiments, each asking a yes-or-no question, and each with its own boolean-valued outcome: a random variable that is true with probability p and false with probability q = 1 − p.

In the context of our system failures, the 'success' can be thought of as the event where a flaw remains in the system at time t. The binomial distribution helps determine the probability of having exactly k remaining flaws. Understanding the binomial distribution is enhanced by exploring simulations or employing graphical illustrations to show how probabilities change with different values of p and sample sizes. This can build a more intuitive grasp of how likely certain outcomes are based on the underlying probabilities of 'success' in each trial.
Random Variables Dependency
Dependency among random variables indicates that the occurrence of certain outcomes of one variable affects the probability of outcomes of another variable. This concept is crucial when we model real-world scenarios where events don't happen in isolation.

In the system failure case, the number of failures and the number of remaining flaws are dependent random variables. This relationship is because the failures directly reduce the number of potential remaining flaws. To elucidate this concept of dependency in instructional materials, contrasting it with independent events where one event has no bearing on another can be valuable. Intuitive examples, such as the link between weather conditions and traffic patterns, can bring clarity to the concept of random variables dependency.

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Most popular questions from this chapter

(a) Let \(\\{N(t), t \geqslant 0]\) be a nonhomogeneous Poisson process with mean value function \(m(t) .\) Given \(N(t)=n\), show that the unordered set of arrival times has the same distribution as \(n\) independent and identically distributed random variables having distribution function $$ F(x)=\left\\{\begin{array}{ll} \frac{m(x)}{m(t)}, & x \leqslant t \\ 1, & x \geqslant t \end{array}\right. $$ (b) Suppose that workmen incur accidents in accordance with a nonhomogeneous Poisson process with mean value function \(m(t)\). Suppose further that each injured man is out of work for a random amount of time having distribution \(F\). Let \(X(t)\) be the number of workers who are out of work at time \(t\). By using part (a), find \(E[X(t)]\).

Customers arrive at a two-server service station according to a Poisson process with rate \(\lambda\). Wheneyer a new customer arrives, any customer that is in the system inmediately departs. A new arrival enters service first with server 1 and then with server 2: If the service times at the servers are independent exponentials with respective rates \(\mu_{1}\) and \(\mu_{2}\), what proportion of entering customers completes their service with server \(2 ?\)

Customers arrive at the automatic teller machine in accordance with a Poisson process with rate 12 per hour. The amount of money withdrawn on each transaction is a random variable with mean \(\$ 30\) and standard deviation \(\$ 50\). (A negative withdrawal means that money was deposited.) The machine is in use

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The number of missing items in a certain location, call it \(X\), is a Poisson random variable with mean \(\lambda\). When searching the location, each item will independently be found after an exponentially distributed time with rate \(\mu\). A reward of \(R\) is received for each item found, and a searching cost of \(C\) per unit of search time is incurred. Suppose that you search for a fixed time \(t\) and then stop. (a) Find your total expected return. (b) Find the value of \(t\) that maximizes the total expected return. (c) The policy of searching for a fixed time is a static policy. Would a dynamic policy, which allows the decision as to whether to stop at each time \(t\), depend on the number already found by \(t\) be beneficial? Hint: How does the distribution of the number of items not yet found by time \(t\) depend on the number already found by that time?

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