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Questions and Answers
What does the Process Capability index (Cpk) measure?
What does the Process Capability index (Cpk) measure?
- The total variation in the output of a process.
- The average output of a process over time.
- The probability of a process producing defective items.
- The consistency of a process in meeting customer specification limits. (correct)
What are the two components compared in calculating the Cpk value?
What are the two components compared in calculating the Cpk value?
- Process mean and specification limits. (correct)
- Range and standard deviation of the process.
- Mean and median of the data set.
- Standard deviation and control limits.
Which situation indicates that a process is considered capable?
Which situation indicates that a process is considered capable?
- Variation of the process exceeds the specification limits.
- All measurements fall outside specification limits.
- Most measurements fall inside specification limits. (correct)
- Control limits exceed the specification limits.
What is a key difference between Cp and Cpk?
What is a key difference between Cp and Cpk?
What does a higher Cpk value indicate?
What does a higher Cpk value indicate?
How is process variation represented in terms of sigma?
How is process variation represented in terms of sigma?
What significant factor is assumed when calculating Cpk?
What significant factor is assumed when calculating Cpk?
What does the rejection percentage analysis evaluate?
What does the rejection percentage analysis evaluate?
What does a higher Cpk value indicate about a process?
What does a higher Cpk value indicate about a process?
Which of the following values of Cpk indicates a process that has a high percentage of rejects?
Which of the following values of Cpk indicates a process that has a high percentage of rejects?
How is Cpk calculated for a process with an upper specification limit (USL) of 18, lower specification limit (LSL) of 0, mean (μ) of 12, and standard deviation (σ) of 3?
How is Cpk calculated for a process with an upper specification limit (USL) of 18, lower specification limit (LSL) of 0, mean (μ) of 12, and standard deviation (σ) of 3?
If a process is centered in its specification limits, which relationship between Cp and Cpk is true?
If a process is centered in its specification limits, which relationship between Cp and Cpk is true?
Which situation would likely result in a higher percentage of rejects?
Which situation would likely result in a higher percentage of rejects?
In which case is the process spread greater than the specification limits?
In which case is the process spread greater than the specification limits?
What will be the effect of increasing the standard deviation while keeping the mean constant in a process?
What will be the effect of increasing the standard deviation while keeping the mean constant in a process?
If the USL is 16 and LSL is 0, what is the Cp for a process with a mean (μ) of 8 and standard deviation (σ) of 4?
If the USL is 16 and LSL is 0, what is the Cp for a process with a mean (μ) of 8 and standard deviation (σ) of 4?
Which condition must be met for the process capability index (Cpk) to be accurately calculated?
Which condition must be met for the process capability index (Cpk) to be accurately calculated?
In the Cpk calculation, which of the following values are used?
In the Cpk calculation, which of the following values are used?
How does a higher standard deviation affect the capability of a process?
How does a higher standard deviation affect the capability of a process?
What does a Cpk value of 1.33 indicate about the process capability?
What does a Cpk value of 1.33 indicate about the process capability?
If the process spread (6σ) is greater than the difference between USL and LSL, what can be inferred?
If the process spread (6σ) is greater than the difference between USL and LSL, what can be inferred?
What is indicated by a percentage of rejects of 0.0064%?
What is indicated by a percentage of rejects of 0.0064%?
What happens to the process capability index if the mean moves closer to the upper specification limit?
What happens to the process capability index if the mean moves closer to the upper specification limit?
When comparing Cp and Cpk, which statement is true?
When comparing Cp and Cpk, which statement is true?
Flashcards
Process Capability Index
Process Capability Index
A measure of a process's ability to meet the specification limits. It's calculated when the process is stable.
Stable Process
Stable Process
A process that is in statistical control and predictable; variations are due to common causes, not special causes.
Specification Limits (USL and LSL)
Specification Limits (USL and LSL)
The maximum (USL) and minimum (LSL) acceptable values for a product or process characteristic.
Cpk
Cpk
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Process Standard Deviation (σ)
Process Standard Deviation (σ)
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Out-of-Control Process
Out-of-Control Process
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Normal Distribution
Normal Distribution
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Capable Process
Capable Process
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Cpk Definition
Cpk Definition
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Higher Cpk Value
Higher Cpk Value
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Cpk = Cp
Cpk = Cp
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Process Spread (6σ)
Process Spread (6σ)
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Cpk Calculation
Cpk Calculation
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Process Capability (Cp)
Process Capability (Cp)
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Process Control
Process Control
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Process Capability
Process Capability
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Process Capability Index (Cpk)
Process Capability Index (Cpk)
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Cpk Value
Cpk Value
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Study Notes
Process Capabilities
- Process capability refers to the uniformity of the process.
- Process capability analysis is an engineering study to estimate the capability of a process.
- Variability in a process measures the uniformity of output.
- Process control evaluates process variation over time to reduce or eliminate variation.
- Process capability evaluates how well a process meets specifications.
- Generally, process capability is used when a process is under statistical control.
- Control limits are functions of natural process variability.
- Natural tolerance limits usually represent 3σ from the mean.
- Specification limits are determined by engineers, developers, designers, managers, clients, etc.
- There is no mathematical relationship between control limits and specification limits.
- Process capability indices (Cp & Cpk) are used to compare process variability to specifications.
- Process sigma value is used to determine process capability.
Process Capability Indices (Cp & Cpk)
- Cp assumes the process mean is at the center of the specification band. It measures potential capability.
- Cpk measures the actual process capability.
- Cpk considers the actual process mean position within the specification limits.
- Cpk = min[(USL–µ)/3σ, (µ–LSL)/3σ]
- Cp = (USL-LSL)/6σ
Major Uses of Process Capability Analysis
- Examine if the level of quality is maintained or improved.
- Assist in product design, selection, and modification.
- Establish an interval between sampling for process monitoring.
- Specifying performance requirements for new equipment or processes.
- Select and control vendors.
- Reduce process variability.
- Process capability = Spread of the process = Dispersion = 6σ
When to Perform Process Capability Analysis
- Process is in statistical control: σ and μ (or X) are constant.
- Process is stable.
- Data is normally distributed.
- Calibration is done.
Interpreting Cp and Cpk
- Larger Cp or Cpk values indicate better process quality.
- Cp values do not change based on the process center.
- Cpk equals Cp only when the process is centered.
- Cpk < Cp indicates the process is not centered.
- Cpk of 1.33 or larger is generally accepted as acceptable, indicating the process meets specification limits.
Steps to Enhance Process Capability
- If Cpk < Cp, shift the process mean (µ) to the true center.
- If Cpk and/or Cp < 1.33, reduce process variability (σ) and check the specifications.
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