Appendix B: Glossary of SPC & Quality Terms

Appendix B

Glossary of SPC & Quality Terms

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A

Assignable Cause (Special Cause):
A source of variation that is identifiable and correctable, arising from specific circumstances such as machine malfunction or operator error.

Acceptance Sampling:
A statistical quality control technique used to decide whether to accept or reject a lot based on a sample.

Attribute Data:
Quality data that are counted or classified, such as number of defects or defectives.

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B

Benchmarking:
The process of comparing performance metrics with industry best practices.

Binomial Distribution:
A probability distribution used for modeling attribute data in p and np charts.

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C

Cause-and-Effect Diagram (Fishbone Diagram):
A graphical tool used to identify potential causes of process variation.

c-Chart:
A control chart used to monitor the number of defects per inspection unit.

Center Line (CL):
The middle line on a control chart representing the process average.

Common Cause Variation:
Natural, random variation inherent in a stable process.

Control Chart:
A graphical tool used to determine whether a process is in statistical control.

Control Limits:
Statistically determined boundaries (UCL and LCL) within which process variation is expected to occur.

Cp (Process Capability Index):
A measure of a process’s potential capability assuming it is centered.

Cpk (Process Capability Index):
A measure of actual process capability considering centering.

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D

Defect:
A nonconformity in a product or service that does not meet quality requirements.

Defective:
A unit containing one or more defects.

DMAIC:
Define-Measure-Analyze-Improve-Control methodology used in Six Sigma.

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E

Empirical Rule:
A statistical rule stating that 99.73% of data in a normal distribution lies within ±3σ.

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F

Failure Mode and Effects Analysis (FMEA):
A systematic method for identifying potential failures and their effects.

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G

Gage R&R:
Measurement system analysis that evaluates repeatability and reproducibility.

Grand Mean (X̄̄):
The average of sample means.

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H

Histogram:
A graphical representation of data distribution.

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I

I–MR Chart (Individuals–Moving Range Chart):
A control chart used when subgroup size is one.

In-Control Process:
A process affected only by common cause variation.

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L

Lower Control Limit (LCL):
The lower statistical boundary of a control chart.

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M

Measurement System Analysis (MSA):
Evaluation of accuracy and precision of measurement systems.

Moving Range (MR):
The absolute difference between successive observations.

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N

Normal Distribution:
A bell-shaped probability distribution commonly assumed in SPC.

np-Chart:
A control chart used to monitor the number of defectives.

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O

Out-of-Control Process:
A process exhibiting special cause variation.

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P

p-Chart:
A control chart used to monitor fraction defective.

Pareto Chart:
A bar chart prioritizing causes based on frequency.

Poisson Distribution:
A probability distribution used for modeling defect counts.

Process Capability:
The ability of a process to produce output within specification limits.

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Q

Quality Control:
Operational techniques used to fulfill quality requirements.

Quality Improvement:
Systematic efforts to enhance process performance.

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R

R-Chart:
A control chart used to monitor process variability using range.

Rational Subgroup:
A group of samples taken under similar conditions.

Repeatability:
Variation due to measurement equipment.

Reproducibility:
Variation due to different operators.

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S

Six Sigma:
A data-driven methodology aimed at reducing defects to 3.4 per million opportunities.

SPC (Statistical Process Control):
The use of statistical methods to monitor and control processes.

Special Cause Variation:
Non-random variation caused by identifiable factors.

Standard Deviation (σ):
A measure of process variability.

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T

Tampering:
Unnecessary adjustment of a stable process, increasing variability.

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U

u-Chart:
A control chart used to monitor defects per unit.

Upper Control Limit (UCL):
The upper statistical boundary of a control chart.

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V

Variation:
The natural differences observed in process output.

Variable Data:
Measured quality characteristics such as length or weight.

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X

X̄-Chart:
A control chart used to monitor process mean.