Problem 150 - Applied Statistics and Probability for Engineers

Question

The total service time of a multistep manufacturing operation has a gamma distribution with mean 18 minutes and standard deviation 6. (a) Determine the parameters \(\lambda\) and \(r\) of the distribution. (b) Assume that each step has the same distribution for service time. What distribution for each step and how many steps produce this gamma distribution of total service time?

Step-by-Step Solution

Answer

Parameters: \( \lambda = \frac{1}{2} \), \( r = 9 \). There are 9 steps, each following an exponential distribution.

Step 1: Understand the Gamma Distribution Parameters

The gamma distribution is characterized by two parameters: the shape parameter \( r \) and the rate parameter \( \lambda \). The mean of the gamma distribution is given by \( \mu = \frac{r}{\lambda} \), and the variance is \( \sigma^2 = \frac{r}{\lambda^2} \). Given that the mean is 18 minutes and the standard deviation is 6 minutes, we know that the variance \( \sigma^2 = 6^2 = 36 \).

Step 2: Set Up the Equations

From the given information, we can set up two equations:1. \( \frac{r}{\lambda} = 18 \) (for the mean)2. \( \frac{r}{\lambda^2} = 36 \) (for the variance)We will use these equations to solve for \( r \) and \( \lambda \).

Step 3: Solve for \( \lambda \) and \( r \)

Let's start by solving the first equation for \( \lambda \):\[ \lambda = \frac{r}{18} \]Substitute \( \lambda \) into the second equation:\[ \frac{r}{\left(\frac{r}{18}\right)^2} = 36 \]This simplifies to:\[ \frac{18^2 \cdot r}{r^2} = 36 \]\[ \frac{324}{r} = 36 \]\[ r = \frac{324}{36} = 9 \]Now substitute \( r = 9 \) back to find \( \lambda \):\[ \lambda = \frac{9}{18} = \frac{1}{2} \]

Step 4: Identify Distribution and Number of Steps for Part (b)

Since the total service time follows a gamma distribution with shape \( r = 9 \), this suggests there are 9 steps, as the shape parameter \( r \) often represents the number of gamma-distributed processes (or steps). Each step also follows an exponential distribution with rate \( \lambda = \frac{1}{2} \), because the sum of gamma distributions with equal rates leads to an exponential distribution.

Key Concepts

Probability Distribution

When we talk about a probability distribution, we're referring to a mathematical function that describes the likelihood of different outcomes in a random experiment. It's a key concept in statistics and is crucial for understanding various data behaviors and predictions.

Gamma distribution is one type of continuous probability distribution. It is particularly useful when modeling waiting times in processes where events occur continuously and independently. For example, it can model the time required for a series of events to occur, like manufacturing steps.

Understanding how probability distributions apply to real-world cycles, such as service or wait times, can be very helpful. Such insights allow for better planning and optimization in queuing and service-related fields.

Shape and Rate Parameters

The gamma distribution heavily relies on two parameters: the shape parameter \( r \) and the rate parameter \( \lambda \). These parameters help govern the distribution's characteristics.

• Shape parameter (\( r \)): This parameter defines the number of stages or the dimensional aspect of the distribution. A higher \( r \) indicates more stages or steps in the process.
• Rate parameter (\( \lambda \)): This determines the rate at which events occur. It's inversely proportional to the mean; hence, higher \( \lambda \) means faster occurrences of events.

Solving problems with the gamma distribution generally involves identifying these parameters using provided statistical data, like mean and standard deviation. In the given exercise, the mean and variance are used to find \( r = 9 \) and \( \lambda = \frac{1}{2} \). These parameters play a crucial role in tailoring the distribution to fit real-life processes like manufacturing.

Exponential Distribution

An exponential distribution is a special case of the gamma distribution. It specifically arises when the shape parameter \( r \) is equal to 1. This distribution deals with the time between events in a process where events occur continuously and independently at a constant average rate.

In the context of the given exercise, each step of the manufacturing process follows an exponential distribution with the rate \( \lambda = \frac{1}{2} \). This is because every individual step's service time can be seen as a distinct event that occurs over time.

The exponential distribution is pivotal in determining the wait time or service time in many real-life processes. Its properties make it a vital tool for modeling scenarios where events are expected to follow at random intervals.

Manufacturing Processes

Manufacturing processes often benefit from statistical analysis to optimize efficiency and reduce downtime. The total time taken for multi-step operations, as in the given exercise, can be modeled using a gamma distribution due to its ability to manage variability in process time.

By breaking down each step to follow an exponential distribution and summing them, one can ascertain the overall time duration using these distributions. For example:

• Understanding total time: Total service time is represented by a gamma distribution with a shape parameter matching the number of steps.
• Individual step ": Each step possessing an exponential distribution helps identify potential bottlenecks or delays in process stages.

Effectively applying these concepts leads to better prediction models for manufacturing operations, ultimately enhancing production flow and ensuring timely completions.