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Statistical Quality Control Versus Statistical Process Control (SQC Versus SPC)

In 1974 Dr. Kaoru Ishikawa brought together a collection of process improvement tools in his text Guide to Quality Control. Known around the world as the seven quality control (7–QC) tools, they are:

  • Cause–and–effect analysis
  • Check sheets/tally sheets
  • Control charts
  • Graphs
  • Histograms
  • Pareto analysis
  • Scatter analysis

In addition to the basic 7–QC tools, there are also some additional tools known as the seven supplemental (7–SUPP) tools:

  • Data stratification
  • Defect maps
  • Events logs
  • Process flowcharts/maps
  • Progress centers
  • Randomization
  • Sample size determination

Statistical quality control (SQC) is the application of the 14 statistical and analytical tools (7–QC and 7–SUPP) to monitor process outputs (dependent variables). Statistical process control (SPC) is the application of the same 14 tools to control process inputs (independent variables). The figure below portrays these relationships.

SQC versus SPC

Design of experiments (DOE) and
analysis of variance (AOV or ANOVA)


Excerpted from Jack B. ReVelle’s Quality Essentials: A Reference Guide from A to Z, ASQ Quality Press, 2004, pages 185–186.

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