Variability Reduction

Variability Reduction Program Business Excellence1

Variability Reduction Variability Reduction ProgramProgramolol


OutlineOutline

Section 1: Understanding the Impact Section 1: Understanding the Impact of Variationof Variation

Section 2: Process Control SystemsSection 2: Process Control Systems

Section 3: Measurement Capability Section 3: Measurement Capability AnalysisAnalysis


What is Variation?What is Variation?

Loss FunctionLoss Function

What is the target for a given process?What is the target for a given process?

How far away from the target is still OK?How far away from the target is still OK?

Target

??


A curve representing loss in quality as A curve representing loss in quality as a function of a measured valuea function of a measured value


Lossin

Quality

Measurement

Loss FunctionLoss Function

target


What is Variation?What is Variation?Traditional “Loss Function”Traditional “Loss Function”

Spec limits define Spec limits define acceptableacceptable and and unacceptableunacceptable quality quality

Lossin

Quality

LowerSpecLimit

UpperSpecLimit


Modern “Loss Function”Modern “Loss Function”

Target: Desired Value or OutcomeTarget: Desired Value or Outcome


Lossin

Quality

Target

Loss in QualityIncreases as

Variation fromTarget Increases


What is Variation?What is Variation?How the Loss Function Affects ProductHow the Loss Function Affects Product

Both are within specBoth are within specWhich is more desirable?Which is more desirable?

LSLLSL USL USL


TargetNumber of Defects

Lossin

Quality

What is Variation?What is Variation?Loss Function for DefectsLoss Function for Defects



Less Variation Less Variation

==

Higher QualityHigher Quality


Examining VariationExamining VariationDefinitionDefinition

A A Stable ProcessStable Process has the has the same normal distribution at same normal distribution at

all times.all times.

A stable process is A stable process is In ControlIn Control

A stable process still has variationA stable process still has variation


Examining VariationExamining VariationStable ProcessStable Process

Normal distribution at all timesNormal distribution at all times

Prediction

Time


Examining VariationExamining Variation

Common CausesCommon Causes

The cause of variations in a The cause of variations in a stable process is called a stable process is called a

Common CauseCommon Cause..

A common cause is a natural cause of A common cause is a natural cause of variation in the system.variation in the system.



Machine vibrationMachine vibration Temperature fluctuationsTemperature fluctuations Slight variation in raw materialsSlight variation in raw materials Human variation in setting control Human variation in setting control

dialsdials

Common Cause ExamplesCommon Cause Examples


Examining VariationExamining VariationTools for Examining StabilityTools for Examining Stability

Trend ChartTrend Chart: A plot showing the : A plot showing the behavior of a process over time.behavior of a process over time.

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Examining VariationExamining VariationTools for Examining StabilityTools for Examining Stability

HistogramHistogram: A barchart showing the : A barchart showing the distribution of the process.distribution of the process.

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Activity: Comparing stable processesActivity: Comparing stable processes

Which process has better quality?Which process has better quality?

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Sequence0 5 10 15 20 25

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Examining VariationExamining VariationActivity: Comparing stable processes Activity: Comparing stable processes

(cont’d)(cont’d)

Which process has better quality?Which process has better quality?

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Sequence0 5 10 15 20 25

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Examining VariationExamining VariationUnstable ProcessUnstable Process

Any process that is not stable is called an Any process that is not stable is called an unstableunstable or or out-of-controlout-of-control process. process.

Prediction

Time

??


Examining VariationExamining VariationKinds of Instability: Kinds of Instability: ExcursionsExcursions

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Kinds of Instability: Kinds of Instability: ShiftsShifts

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Kinds of Instability: Kinds of Instability: DriftsDrifts

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Kinds of Instability: Kinds of Instability: CyclesCycles

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Kinds of Instability: Kinds of Instability: ChaosChaos

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Special CausesSpecial Causes

Anything that causes variations that Anything that causes variations that are not part of the stable process are not part of the stable process

is called a is called a special causespecial cause, , assignable causeassignable cause, or , or unnatural unnatural

causecause..



Batch of defective raw materialBatch of defective raw material Faulty set-upFaulty set-up Human errorHuman error Incorrect recipeIncorrect recipe Blown gasketBlown gasket EarthquakeEarthquake

Examples of Special CausesExamples of Special Causes


Reducing VariationReducing Variation

Centering at TargetCentering at Target

Reducing Common Cause VariationReducing Common Cause Variation

Improving a Stable ProcessImproving a Stable Process

Two strategies for improving a stable processTwo strategies for improving a stable process


Reducing VariationReducing VariationCentering at TargetCentering at Target

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Reducing VariationReducing VariationReducing Common Cause VariationReducing Common Cause Variation

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Structured problem solvingStructured problem solving Planned experimentsPlanned experiments

Reducing Variation in a Stable Reducing Variation in a Stable ProcessProcess

Make Permanent ChangesMake Permanent Changes

Changes are based on the scientific Changes are based on the scientific approachapproach

Examples: new equipment, equipment upgrade, new procedure, new machine

settings, better raw material



Do not Do not ignore special causes.ignore special causes. DoDo quickly detect special cause quickly detect special cause

variations.variations. DoDo stop production until the process stop production until the process

is fixed. (Reactive)is fixed. (Reactive) DoDo identify and permanently identify and permanently

eliminate special causes. (Preventive)eliminate special causes. (Preventive)

Reducing Variation in an Reducing Variation in an Unstable ProcessUnstable Process



DetectDetect the special cause variation. the special cause variation. IdentifyIdentify the special cause. the special cause. FixFix the process the process

• Remove the special cause, orRemove the special cause, or• Compensate for the special cause.Compensate for the special cause.

PreventPrevent the special cause from the special cause from occurring againoccurring again

Improving an Unstable ProcessImproving an Unstable Process

Four Step ProcessFour Step Process


Reducing VariationReducing VariationImproving an Unstable ProcessImproving an Unstable Process

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Detect HereNot Here

Detect Here


Reducing VariationReducing VariationImproving an Unstable ProcessImproving an Unstable Process

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Unstable Stable


Detecting VariationDetecting Variation

How can we decide if How can we decide if variation is the result of variation is the result of

common or special cause?common or special cause?


Detecting VariationDetecting VariationTool: Control ChartTool: Control Chart

Benefit: Prevents tampering or ignoringBenefit: Prevents tampering or ignoring

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Detecting VariationDetecting VariationControl Chart for Detecting VariationControl Chart for Detecting Variation

ObserveVariation

CommonCause

DetectSpecial Cause

Identify

Fix

Prevent

Don’tTamper

Reduce OverallVariation

ControlControlChartChart


Detecting VariationDetecting VariationControl Chart for Detecting VariationControl Chart for Detecting Variation

Control ChartControl Chart

Trend ChartTrend Chart Center LineCenter Line Control LimitsControl Limits++ ++

Upper Control Limit

Lower Control Limit

Center Line


Detecting VariationDetecting VariationControl LimitsControl Limits

Control limits tell us where the Control limits tell us where the measurements in a stable process measurements in a stable process

should fallshould fallLowerControlLimit

UpperControlLimit


Detecting VariationDetecting VariationControl LimitsControl Limits

Calculated statistically, based on:Calculated statistically, based on:– Historical DataHistorical Data– Characteristics of the stable processCharacteristics of the stable process

Also called Also called 6 sigma6 sigma limits limits

HighlyUnlikely

1.5 out of1000

HighlyUnlikely

1.5 out of1000

+3-3


Detecting VariationDetecting VariationCreating a Control ChartCreating a Control Chart

Turn the distribution on its sideTurn the distribution on its side

Upper Control Limit

Lower Control Limit

Center Line



Can we use spec limits as Can we use spec limits as control limits?control limits?

Can we compute control limits Can we compute control limits for an unstable process?for an unstable process?



What is the Center Line?What is the Center Line?

Process mean, based on historical Process mean, based on historical datadata

oror

Process TargetProcess Target

Creating a Control ChartCreating a Control Chart



Creating a Control ChartCreating a Control Chart

Selecting the Center LineSelecting the Center Line

The center line should be the The center line should be the target, unless we are target, unless we are unableunable or or unwilling to control the process unwilling to control the process to target.to target.

Since the target is zero defects, Since the target is zero defects, the center line is the process the center line is the process mean.mean.

Measurements:Measurements:

Defects:Defects:



Control LimitsControl Limits Based on Based on

performance of performance of the the processprocess..

Tell us when to Tell us when to take action on take action on the the processprocess..

Spec LimitsSpec Limits Based on Based on

performance of performance of the the productproduct..

Tell us when to Tell us when to disposition the disposition the productproduct..

Control Limits vs. Spec LimitsControl Limits vs. Spec Limits



Focus OnFocus On

Control LimitsControl Limits

Improve Process Improve Process QualityQuality

Spec LimitsSpec Limits

Improve Product Improve Product QualityQuality

Control Limits vs. Spec Limits



On-LineOn-Line: Assess the present : Assess the present stability of a process, as part of a stability of a process, as part of a process control system process control system (PCS)(PCS)

Off-LineOff-Line: Assess the historical : Assess the historical stability of a processstability of a process

Uses of a Control ChartUses of a Control Chart


What is a PCS?What is a PCS?A PCS is a subset of the A PCS is a subset of the

Reduction in Variation flowReduction in Variation flow

ObserveVariation

CommonCause

Don’tTamper

Reduce OverallVariation

ControlControlChartChart

DetectSpecial Cause

Identify

Fix

Prevent

Documents

Variability Reduction