Process Capability Analysis Overview

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Questions and Answers

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?

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?

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?

Cp measures the overall process variation, while Cpk accounts for process mean shift. Signup and view all the answers

What does a higher Cpk value indicate?

Higher probability of meeting specifications. Signup and view all the answers

How is process variation represented in terms of sigma?

Process spread is equal to 6σ. Signup and view all the answers

What significant factor is assumed when calculating Cpk?

The population of data values is normally distributed. Signup and view all the answers

What does the rejection percentage analysis evaluate?

The effectiveness of a process in producing acceptable products. Signup and view all the answers

What does a higher Cpk value indicate about a process?

The process has a higher capability to meet specifications. Signup and view all the answers

Which of the following values of Cpk indicates a process that has a high percentage of rejects?

0.67 Signup and view all the answers

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?

min[(18-12)/3, (12-0)/3] Signup and view all the answers

If a process is centered in its specification limits, which relationship between Cp and Cpk is true?

Cp = Cpk Signup and view all the answers

Which situation would likely result in a higher percentage of rejects?

Cpk of 0.67 Signup and view all the answers

In which case is the process spread greater than the specification limits?

Case 3-L Signup and view all the answers

What will be the effect of increasing the standard deviation while keeping the mean constant in a process?

Cpk will decrease. Signup and view all the answers

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?

0.67 Signup and view all the answers

Which condition must be met for the process capability index (Cpk) to be accurately calculated?

The process must be in statistical control. Signup and view all the answers

In the Cpk calculation, which of the following values are used?

The upper and lower specification limits and the mean. Signup and view all the answers

How does a higher standard deviation affect the capability of a process?

It limits the process ability to meet specification limits. Signup and view all the answers

What does a Cpk value of 1.33 indicate about the process capability?

The process is highly capable. Signup and view all the answers

If the process spread (6σ) is greater than the difference between USL and LSL, what can be inferred?

The process needs improvement. Signup and view all the answers

What is indicated by a percentage of rejects of 0.0064%?

A process that produces few non-conforming products. Signup and view all the answers

What happens to the process capability index if the mean moves closer to the upper specification limit?

Cpk will decrease. Signup and view all the answers

When comparing Cp and Cpk, which statement is true?

Cp measures potential capability, while Cpk measures actual performance. Signup and view all the answers

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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Description

This quiz covers the fundamentals of process capability and its analysis in engineering. It explores topics such as variability, process control, specification limits, and process capability indices like Cp and Cpk. Test your understanding of how these concepts influence the quality of processes.