A p-chart would be used to monitor
Multiple Choice
A the number of defects per unit.
B the range of values.
C the fraction defective.
D average shrinkage.
E dispersion in sample data.
The correct answer and explanation is :
The correct answer is C) the fraction defective.
Explanation:
A p-chart (proportion chart) is a type of control chart used in statistical process control (SPC) to monitor the proportion of defective items in a sample over time. It is particularly useful when dealing with categorical data, where each item is classified as either defective or non-defective, often referred to as a “pass” or “fail.” The p-chart is used to track the fraction defective in a process, which helps assess the consistency and stability of that process.
The p-chart plots the proportion of defective units (or the fraction defective) in each sample on the vertical axis, while the time or sample number is plotted on the horizontal axis. The chart typically includes a central line representing the average proportion defective, as well as upper and lower control limits, which are calculated based on the variability of the process. When the proportion of defects falls outside these control limits, it signals that the process may be out of control and may require corrective action.
Key Points about the p-chart:
- Purpose: It monitors the fraction defective in a process. This is useful in quality control when the goal is to keep the defect rate within an acceptable range.
- Applications: p-charts are used in various industries such as manufacturing, healthcare, and service industries to monitor product quality. For instance, a factory producing electronic devices may use a p-chart to monitor the proportion of defective products in each batch.
- Process Monitoring: The p-chart allows quality control personnel to detect trends, shifts, or sudden increases in defect rates, ensuring early intervention before defects become widespread or costly.
In contrast:
- A) The number of defects per unit would typically be monitored using a c-chart (count of defects per item).
- B) The range of values is often monitored using an R-chart, which tracks the variability within a sample.
- D) Average shrinkage (often related to inventory loss) would require different types of analysis, such as a loss function model or a specific inventory management chart.
- E) Dispersion in sample data would be analyzed using an R-chart or an S-chart, which focus on the spread or variability in sample values.
Thus, the p-chart is specifically designed for monitoring the proportion of defective items, making C) the fraction defective the correct answer.