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The number of cracks in a section of interstate highway that are significant enough to require repair is assumed to follow a Poisson distribution with a mean of two cracks per mile. (a) What is the probability that there are no cracks that require repair in 5 miles of highway? (b) What is the probability that at least one crack requires repair in \(1 / 2\) mile of highway? (c) If the number of cracks is related to the vehicle load on the highway and some sections of the highway have a heavy load of vehicles whereas other sections carry a light load, how do you feel about the assumption of a Poisson distribution for the number of cracks that require repair?

Short Answer

Expert verified
(a) \( e^{-10} \). (b) \( 1 - e^{-1} \). (c) Non-constant rates challenge Poisson model assumptions.

Step by step solution

01

Understanding the Poisson Distribution

A Poisson distribution models the number of events occurring in a fixed interval of time or space. The mean, denoted by \( \lambda \), is the expected number of events. In this problem, \( \lambda = 2 \) cracks per mile, and we need to adjust \( \lambda \) based on the different distances specified in parts (a) and (b).
02

Calculate for 5 Miles with No Cracks

For part (a), we want the probability of 0 cracks in 5 miles. The rate \( \lambda \) for 5 miles is \( 5 \times 2 = 10 \). The probability of 0 events (cracks) is given by \( P(X=0) = \frac{e^{-\lambda} \lambda^0}{0!} = e^{-10} \). Calculate this probability.
03

Determine the Probability for At Least One Crack in 1/2 Mile

Part (b) requires finding the probability of at least 1 crack in 1/2 mile. First calculate the rate: \( \lambda = 0.5 \times 2 = 1 \). Then find the probability of no cracks \( P(X=0) = e^{-1} \). The probability of at least one crack is \( 1 - P(X=0) = 1 - e^{-1} \).
04

Evaluate the Assumption of Poisson Distribution

In part (c), assess the assumption of the Poisson distribution. Since the Poisson distribution assumes a constant rate \( \lambda \) over the interval, if vehicle loads vary significantly between highway sections, the assumption may not strictly hold due to changes in \( \lambda \). However, it could still be used as an approximation if variations are not extreme.

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Key Concepts

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

probability calculation
Probability calculation in the context of the Poisson distribution involves determining the likelihood of a specific number of events occurring within a given time or space interval. In this exercise, the Poisson distribution is applied to model the number of cracks in highway sections. The core of Poisson distribution probability calculations is the formula:
  • The rate per interval, \( \lambda \), which is the expected number of occurrences (e.g., cracks per mile).
  • The exponential function \( e \) raised to the power of negative \( \lambda \), \( e^{-\lambda} \).
  • The observed number of occurrences, represented by a factorial, in the denominator \( x! \).
To compute the probability of having a certain number of events like no cracks in 5 miles, we adjust the rate by multiplying the rate per mile by the number of miles, resulting in an effective \( \lambda = 10 \) for 5 miles. The probability of zero cracks \( P(X=0) \) is given by the formula:\[P(X=0) = \frac{e^{-10} \times 10^0}{0!}\]To find probabilities relating to events like at least one crack in 0.5 miles, calculate the probability of zero events first and subtract it from 1. It's essential to be careful with units and accurately interpret what these probabilities mean in real-world scenarios.
engineering statistics
Engineering statistics often leverage probability models like the Poisson distribution to analyze and make informed decisions about engineering problems. Within this exercise context, such statistical models allow engineers to predict the probability of occurrences such as cracks that need repairs, which is crucial for maintenance planning and safety.
If the analysis determines a low probability of significant cracks in a longer distance (e.g., 5 miles), engineers might prioritize other sections of the highway. Conversely, a high probability of at least one crack in shorter intervals (e.g., 0.5 miles) can alert engineers to monitor these sections more closely.
  • This application ensures cost-effective and timely repair work.
  • Contributes to road safety by predicting structural weaknesses.
  • Allows streamlined allocation of resources across highway sections.
In practice, these calculations assist in building robust criticism if assumptions about vehicle load unevenness could render the model less reliable. Engineers must remain aware of the potential limitations of the Poisson model when external factors like varying traffic loads introduce deviations from constant rates \( \lambda \). However, engineers can still use the Poisson model effectively when variations are minor.
statistical modeling
Statistical modeling with the Poisson distribution enables us to understand events happening independently in a fixed interval, like cracks appearing over highway sections. This modeling becomes a powerful tool, especially in cases like highway planning and infrastructure stability.
In this exercise, the Poisson model's assumption is that cracks appear randomly across consistent vehicle load sections. Situations where conditions deviate may lead to incorrect assumptions rendering the Poisson distribution less applicable.
  • The Poisson process assumes a uniform rate, which might not match reality if loads vary significantly.
  • This flexibility in modeling simple processes makes it impactful and widely useable.
  • Models need invalid assumptions analyzed explicitly when designing or maintaining infrastructure.
Ultimately, while Poisson statistical modeling offers a streamlined way to predict and plan for crack maintenance, it's important for engineers to consider real-world variations and adjust models as necessary. This methodology guides decision-making, emphasizing the importance of matching models closely with conditions.

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