Ch05 Control Charts

8/7/2019 Ch05 Control Charts

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Control Charts

Review homework

Ch 4 8, 10, 24, 26, 36,

44, 49

Lecture/discussion

Variable control charts

SPC at Maine Medical

Week 10 assignment

Homework

Ch 5 2, 8, 18, and

control chart

handout

Read Chapters 6 and 7

Process Capability

Other Variable

Control Charts

Week 9AgendaAgenda


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Control Charts

Control ChartsControl Charts

Chapter Five


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Control Charts

Control chartfunctionsControl chartfunctions

Control charts are decision-making tools- they provide an economic basis fordeciding whether to alter a process orleave it alone

Control charts are problem-solving tools -they provide a basis on which to

formulate improvement actions SPC exposes problems; it does not solve

them!


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Control Charts

Control chartsControl charts

Control charts are powerful aids to

understanding the performance of a

process over time.

PROCESSPROCESS

Input Output

Whats causing variability?


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Control Charts

Control chartsidentifyControl chartsidentify

variationvariation Chance causes - common cause

inherent to the process or random and not

controllable if only common cause present, the process

is considered stable or in control

Assignable causes - special cause

variation due to outside influences

if present, the process is out of control


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Control Charts

Control charts help us learnControl charts help us learn

moreaboutprocessesmoreaboutprocesses Separate common and special causes of

variation

Determine whether a process is in a stateof statistical control or out-of-control

Estimate the process parameters (mean,

variation) and assess the performance ofa process or its capability


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Control Charts

Control chartsto monitorControl chartsto monitor

processesprocesses To monitor output, we use a control chart

we check things like the mean, range,

standard deviation To monitor a process, we typically use

two control charts

mean (or some other central tendencymeasure)

variation (typically using range or standard

deviation)


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Control Charts

Control chartcomponentsControl chartcomponents

Centerline

shows where the process average is

centered or the central tendency of the data Upper control limit (UCL) and Lower

control limit (LCL)

describes the process spread


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Control Charts

Control chartforvariables (ChControl chartforvariables (Ch

5)5) Variables are the measurablemeasurable

characteristics of a product or service.

Measurement data is taken and arrayedon charts.


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Control Charts

XX--barandR chartsbarandR charts

The X-bar chart - used to detect changes

in the mean between subgroups

tests central tendency or location effects The R chart - used to detect changes in

variation within subgroups

tests dispersion effects


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Control Charts

Step 1 Definethe problemStep 1 Definethe problem

Use other quality tools to help determine

the general problem thats occurring and

the process thats suspected of causingit.

brainstorm using cause and effect diagram,

why-why, Pareto charts, etc.


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Control Charts

Step 2Selecta quality characteristicStep 2Selecta quality characteristic

to be measuredto be measured Identify a characteristic to study - for

example, part length or any other

variable affecting performance typically choose characteristics which are

creating quality problems

possible characteristics include: length,

height, viscosity, color, temperature,

velocity, weight, volume, density, etc.


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Control Charts

Step 3 ChooseasubgroupStep 3 Chooseasubgroup

sizeto besampledsizeto besampled Choose homogeneous subgroups

Homogeneous subgroups are produced

under the same conditions, by the samemachine, the same operator, the same

mold, at approximately the same time.

Try to maximize chance to detect

differences between subgroups, while

minimizing chance for difference with a

group.


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Control Charts

OtherguidelinesOtherguidelines

The larger the subgroup size, the more

sensitive the chart becomes to small

variations.

This increases data collection costs.

Destructive testing may make large subgroup

sizes infeasible.

Subgroup sizes smaller than 4 arentrepresentative of the distribution averages.

Subgroups over 10 should use S chart.


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Control Charts

Step 4 CollectthedataStep 4 Collectthedata

Run the process untouched to gather

initial data for control limits.

Generally, collect 20-25 subgroups (100total samples) before calculating the

control limits.

Each time a subgroup of sample size n istaken, an average is calculated for the

subgroup and plotted on the control

chart.


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Control Charts

Step 5 DeterminetrialStep 5 Determinetrial

centerlinecenterline The centerline should be the population

mean, Q

Since it is unknown, we use X doublebar, or the grand average of the

subgroup averages.

m

m

i

i!! 1X

X


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Control Charts

Step 6 Determinetrial controlStep 6 Determinetrial control

limitslimits-- X

barchartXbarchart

The normal curve displays the

distribution of the sample averages.

A control chart is a time-dependentpictorial representation of a normal curve.

Processes that are considered under

control will have 99.73% of their graphedaverages fall within six standard

deviations.


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Control Charts

UCL LCL calculationUCL LCL calculation

deviationstandard

3XLCL

3XUCL

!

!

!

W

W

W


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Control Charts

Determininganalternative value forDetermininganalternative value for

thestandarddeviationthestandarddeviation

m

m

i

i

!! 1R

R

RAXUCL 2!

RAXLCL2

!


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Control Charts

Step 7 Determinetrial controlStep 7 Determinetrial control

limitslimits--

R chartR chart The range chart shows the spread or

dispersion of the individual samples

within the subgroup. If the product shows a wide spread, then

the individuals within the subgroup are not

similar to each other.

Equal averages can be deceiving.

Calculated similar to x-bar charts;

Use D3 and D4 (appendix 2)


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Control Charts

RR--barchartexceptionsbarchartexceptions

Because range values cannot be

negative, a value of 0 is given for the

lower control limit of sample sizes of sixor less.


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Control Charts

Step 8 Examinethe processStep 8 Examinethe process--Interpretthe charts

Interpretthe charts A process is considered to be stable

and in a state of control, or undercontrol, when the performance of the

process falls within the statistically

calculated control limits and exhibits

only chance, or common causes.


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Control Charts

Consequences ofConsequences of

misinterpretingthe processmisinterpretingthe process Blaming people for problems that they cannot

control

Spending time and money looking for problems

that do not exist

Spending time and money on unnecessary

process adjustments

Taking action where no action is warranted

Asking for worker-related improvements when

process improvements are needed first


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Control Charts

Process variationProcess variation

When a system is subject to only chance

causes of variation, 99.73% of the

measurements will fall within 3 standarddeviations

If 1000 subgroups are measured, 997 will

fall within the six sigma limits.


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Control Charts

ChartzonesChartzones

Based on our knowledge of the normal curve, a

control chart exhibits a state of control when:

Two thirds of all points are near the center

value.

The points appear to float back and forth

across the centerline.

The points are balanced on both sides of thecenterline.

No points beyond the control limits.

No patterns or trends.


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Control Charts

Identifying patternsIdentifying patterns

Trends

steady, progressive changes in level

Change, jump, or shift in level Runs - 7 points above or below; six

increasing or decreasing, clusters

Recurring cycles Two populations

Mistakes


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Control Charts

Step 9 Revisethe chartsStep 9 Revisethe charts

In certain cases, control limits are revised

because:

out-of-control points were included in thecalculation of the control limits.

The process is in-control but the within

subgroup variation significantly improves.


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Control Charts

Revisingthe chartsRevisingthe charts

Interpret the original charts

Isolate the causes

Take corrective action Revise the chart

Only remove points for which you can

determine an assignable cause


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Control Charts

Step 10Achievethe purposeStep 10Achievethe purpose

Our goal is to decrease the variation

inherent in a process over time.

As we improve the process, the spread ofthe data will continue to decrease.

Quality improves!!

Ch05 Control Charts

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