Question

Which of the following control chart is used for the number of defects in a piece or in a sample?

• A. R-Chart
• B. $\bar{X}$ - Chart
• C. $p$ - Chart
• D. $C$-Chart

Hint:

Remember the "C" connection: C-Chart is for Counting the number of individual defects. Use the $p$-Chart when you are looking for the proportion (percentage) of items that are bad overall.

Answer 1

The Correct Option is D

Control charts are essential tools in Statistical Quality Control (SQC) used to monitor whether a process is in a state of statistical control. They are broadly categorized into two types: Variables charts and Attributes charts. 1. Variable Control Charts: Charts like the $\bar{X}$-chart (Mean chart) and R-chart (Range chart) are used for measurable characteristics like length, weight, or diameter. They track continuous data.

2. Attribute Control Charts: These are used for characteristics that can be counted rather than measured.

• $p$-Chart: Used to monitor the fraction or proportion of defective items in a sample.

• $C$-Chart: Specifically designed to monitor the actual

number of defects in a single unit or a fixed sample size. It assumes that the opportunity for a defect is large while the actual occurrence is small, following a Poisson distribution.

3. Application of the $C$-Chart: The $C$-chart is ideal for complex products where a single item might have multiple defects, such as the number of surface scratches on a refrigerator or the number of missing rivets on an aircraft wing. Therefore, for counting the total number of defects in a specific sample or piece, the $C$-chart is the standardized choice in industrial quality engineering.