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In the field of quality control the science of statistics is often used to determine if a process is "out of control." Suppose the process is, indeed, out of control and \(20 \%\) of items produced are defective. (a) If three items arrive off the process line in succession, what is the probability that all three are defective? (b) If four items arrive in succession, what is the probability that three are defective?

Short Answer

Expert verified
The probability that all three are defective is 0.8%. The probability that three out of four items are defective is 2.56%.

Step by step solution

01

Calculating the Probability of All Three Being Defective

In the first case, we are looking at three successive events, all of which we want to be 'successful' (or in our case, defective). This falls under the rule that the product of probabilities in successive independent events is equal to the total probability of these events occurring in order. Therefore, we calculate \(0.20 \times 0.20 \times 0.20 = 0.008\). This implies 0.008 or 0.8% chance that all three successive items will be defective.
02

Calculating the Probability of Exactly Three out of Four Being Defective

In the second case, we are looking for a specific number (three) of 'successes' (defective items) in a row of four events. This is a task for the binomial probability formula, which is defined as \(P(X=k) = C(n, k) \times p^{k} \times (1-p)^{n-k}\), where 'n' is the total number of events ('items' in our case), 'k' the number of desired 'successes' (defective items, three in our case). 'p' denotes the single event probability of success (item is defective, 20% or 0.20 in our case). The symbol \(C(n, k)\) represents the number of combinations of n items taken k at a time. Computing this we get \(P(X=3) = C(4, 3) \times (0.20)^3 \times (0.80)^1 = 4 \times 0.008 \times 0.8 = 0.0256\). Therefore, there's a 0.0256 or 2.56% chance that exactly three out of the four successive items will be defective.

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Key Concepts

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

Binomial Probability Formula
Understanding the binomial probability formula is crucial for various applications in statistics, particularly in quality control. In essence, it allows us to compute the likelihood of observing a specific number of 'successes' across a series of independent trials, each with the same probability of success.

Let's break down the formula:
The probability of \'k\' successes in \'n\' trials is given by
\[ P(X=k) = C(n, k) \times p^{k} \times (1-p)^{n-k} \]
Here,
  • \( P(X=k) \) is the probability of having exactly \'k\' successes.
  • \( C(n, k) \) refers to the binomial coefficient, representing the number of ways to choose \'k\' successes out of \'n\' trials.
  • \( p \) is the probability of success on a single trial.
  • \( (1-p) \) is the probability of failure.

This formula is derived from the principles of combinatorics and the basic rules of probability. In the context of quality control, 'success' might paradoxically represent a defective item, and the formula helps us understand the consistency of defects, which is pivotal in process improvement efforts.
Quality Control Statistics
When it comes to ensuring product quality, statistical methods become indispensable. Quality Control Statistics involve the use of data-driven techniques to monitor, control, and improve the manufacturing process. Key tools include hypothesis testing, control charts, and, as illustrated in the provided textbook example, the binomial probability formula.

These statistics enable professionals to make informed decisions on whether a process is operating within acceptable limits. Specifically, being able to calculate the probability of defects helps identify when a process might be 'out of control' and requires intervention. For instance, knowing that the probability of three items being defective in succession is as high as 0.8% might signal a need for immediate assessment and corrective action in the manufacturing line.
Process Control
Process control in manufacturing is about maintaining the desired output in a consistent and predictable manner. Statistical probability, as part of process control, enables businesses to predict the likely occurrence of events affecting quality.

Detailed within the probabilities that we calculate using the binomial formula, we can discern patterns or anomalies in production. If a high probability of defects is observed, it suggests deficiencies in the process control system. Effective process control usually leads to reduced variability in products and a higher overall quality.

By calculating the probability for different scenarios—like the likelihood of multiple defects in a row—companies can preemptively adjust processes, thus minimizing waste and maximizing efficiency. Understanding and applying probability in process control is essential for maintaining a competitive edge in the manufacturing industry by ensuring high standards are met consistently.

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