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What is SPC?

What is Statistical Process Control?

SPC stands for Statistical Process Control. SPC includes a series of tools to methodically monitor and control manufacturing processes and also may be applied throughout many other processes in an organization. The way shop-floor statistics (X Bar R charts, etc.) are conveyed and analyzed.

Learn More about SPC with this Tutorial:

SPC Tutorial

SPC stands for Statistical Process Control.

  • Primarily deals with studying variation to improve process performance in any area or process
  • Focuses on prevention, not detection

Fundamental Issues

  • Understanding of common vs. special cases of variation
  • Understand the concept of process capability vs. process control
  • Measurement system must be under control (MSA)

SPC is one of five Core Tools


  • When the variation is only due to common causes, the process is said to be stable
  • A stable process has predictable variation
  • A stable process is also called a process in Control
  • Control Charts are used to study stability

SPC - Variation

A process is in-control (stable) if:

  • There is no variation in the location of the center of the distribution of output over time
  • There is no variation in measures of the dispersion (spread) of the distribution of output over time


A process is out-of-control (unstable) if:

  • There is variation in the location of the center of the distribution of output over time
  • There is variation in measures of the dispersion(spread) of the distribution of output over time

Causes of Variation - Examples

Common Causes of Variation

  • Usual traffic on road
  • Usual play in the slides of a machine
  • Human attentiveness
  • Variation in dimensions in a lot manufactured together

Special Causes of Variation

  • Accident on road
  • Excess play in slide due to wear over time
  • Illness
  • Variation due to change in lot or supplier

SPC - Causes of Variation

Common-cause variation is always present in a process; a process is considered stable and predictable when only this variation is present


Special-cause variation is caused by unusual or external events not related to the common causes of variation and make the process unpredictable and unstable

Capability vs. Stability

If a process is out of control, first we need to stabilize the process


Capability is how well a process performs


Capability has a meaning only when a process is stable

Improve Capability

The first job of a quality specialist is to identify and remove special causes of variation


Once the process is made stable, the next job is to identify the causes of inherent variation and remove them


Improvement in capability can be made only when the process is stable

Capability vs Control

Some statistical tools used

  • X-bar, R-charts
  • Capability (Cpk, Ppk)
  • Run charts
  • Cause & Effect Analysis
  • Affinity
  • Histogram
  • Pareto
  • Scatter Diagram
  • Radar
  • Force field Analysis

Linkages to ISO 16949:2016

  • Identification of Statistical Tools
  • Application Statistical Concepts
  • Control Plan
  • Measurement System Analysis
  • Customer Specific Requirements
  • Core Tools:  APQP, PPAP, SPC, MSA

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