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Six Sigma DMAIC Process - Control Phase - What are Control Charts?

A control chart follows a process statistic over time and compares it with an established baseline. It helps us detect signals of instability; it does not identify the cause by itself. In a conventional three-sigma chart, each side of the centerline is divided into Zones C, B and A. The diagram below shows those bands.

Control-chart zonesSix one-sigma bands around the centerline: C within one sigma, B between one and two, A between two and three. Upper and lower control limits are at plus and minus three sigma in this schematic. These are not specification limits.Control-chart zonesA conventional three-sigma chart of the plotted statisticZone AZone BZone CZone CZone BZone A-3σ-2σ-1σ0σ+1σ+2σ+3σUpper control limitCenterlineMean in this illustrationLower control limitSigma describes variation of the plotted statistic; for subgroup means this differs from individual spread.Control limits describe the baseline process. They are not product specification limits.
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Six Sigma Control Chart Zones


Common-cause and special-cause variation
Characteristics of Common Causes:
  • Arise from the routine influences built into the process.
  • Often have small individual effects that combine into the baseline spread.
  • Produce the stable pattern of random variation described by the baseline model.
  • May still produce too much variation to meet requirements; improvement can require changing the process itself.
Characteristics of Special Causes:
  • Arise from specific influences that are not part of the established stable baseline.
  • Can originate inside or outside the process.
  • Can create large or small changes in level, spread or pattern.
  • Need investigation and an appropriate response to the identified influence.
Common causes and special-cause signalsA stable baseline defines the expected common-cause variation. Out-of-limit points or nonrandom patterns within limits can signal possible special causes. Investigate before assigning a cause; control limits are not product specifications.Common causes and special-cause signalsUse signals to guide investigation; a chart does not name the causeUpper control boundProcess meanLower control boundExpected common-cause variationunder a stable baselineInvestigate possible special-cause signalsInvestigate possible special-cause signalsProcess variationTimeNonrandom patterns within the bounds may also signal. A signal is not proof of a particular cause.A stable process may still fail specifications; common-cause variation is not automatically acceptable.
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Six Sigma Common Causes and Special Causes




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