Continuous Process Improvement

Variation and Mistake-Proofing

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Variation & Mistake Proofing

Upon completing this module, students should be able to:

• Understand common cause and special cause variation

• Understand how to interpret control charts • Understand the Improvement Hierarchy• Understand the concept of Mistake Proofing • Recognize error-proofing in a process

Understanding Variation

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Understanding Variation

One of the most important concepts to understand in process improvement is variation.

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What does variation mean to customers?• Variation means that a process does not

produce the same result (the “Y”) every time.

• Variation directly affects the customer experience!!!.

• Customers “feel” the variation in a process…

Understanding Variation

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Why should we manage our process bymeasuring variation instead of averages?

Understanding Variation

-10

-5

0

5

10

15

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Customers feel the variation not the average!

They feel

this… …not this!!!

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Understanding Variation

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Types of Variation

To successfully interpret variation, you mustknow the difference between:common cause variation

&special cause variation.

"If I had to reduce my message to management to just a few words, I'd say it all has to do with reducing variation.“ - W. Edwards Deming

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Types of Variation

Common Cause Variation • Variation that is inherent in the process. • It is produced by the interactions among the variables of the process. • This collection of variables and their interaction is called the “system of common causes”.• Process improvement teams gather data to understand the common cause system and improve processes.

Special Cause Variation • Variation in the process that is assignable to a specific cause or causes. • This variation arises because of special circumstances. • Process improvement teams may work to mitigate or minimize special cause variation.

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Responding to Process Variation

COMMON CAUSE VARIATION

Actions to take: • Understand that the process has an inherent capability which will not change unless the process is changed.• Identify the process variables that contribute to the common cause variation.• Identify potential ways to change the process.

Actions to avoid: • Doing nothing at all.• Trying to interpret individual occurrences of the process or to explain the difference between those that are high and low.

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Responding to Process Variation

SPECIAL CAUSE VARIATION

Actions to take: • Immediately try to understand when a special cause occurred.• Determine what was different when the special cause occurred. • Identify ways to prevent the special cause from recurring.

Actions to avoid: • Tampering or responding to change in a process and taking action

without understanding the nature of the variation in the process. • Doing nothing at all.• Making fundamental changes in the process.

Visualizing Variation

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Interpreting Control Charts

UCL

CL

LCL

3.Trending

1. Point outside the limitUCL

CL

LCL

UCL

CL

LCL

2. Run

A Control Chart is a tool we can use to identify types of variation.

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Interpreting Control Charts

UCL

CL

LCL

5.Cycling (periodicity)

4. Approaching the center lineUCL

CL

LCL

CL6. Approaching control limits

UCL

LCL

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Sources of Variation

• Common Cause (Random) Variation– Seen as normal movement on the chart– Represent normal operation of the process (good or bad)– Improvement requires management action– Usually represents 85% of variability in a process

• Special Cause (Assignable) Variation– Shown as “out of control” conditions– Represent abnormality in the process– Can be reduced or eliminated through local actions– Usually highly identifiable!!

Variation Exercise

Write the letter 'a' eight times… What do you see? Are they all identical? Why not?

What caused the variation?- Pen used ?- Uneven paper ?- Caffeine level of the writer ?- Angle changed as you wrote ?

What type of variation is this?????

Would it make sense to circle one of the a's and ask: "Why is it different?" No! That particular 'a' is part of a system that includes all the reasons listed above (and more!). This is common cause variation. It is variation that is part of the system. Variation is inherent in all processes!

Variation ExerciseWrite 3 a's with your dominant hand, 3 more with your non-dominant

hand, and then 2 more with your dominant hand.…

What do you see? Are they all identical? Why not?

What caused the variation this time?

What type of variation is this?????

Which ones are obviously different? You can circle the radically different a 's and say, "Something out of the ordinary occurred here!" (The answer is that you switched your hand.) This variation is called special cause variation.

Common Cause Variation

Special Cause variation

You catch your “favorite” light…it delays you by 2 minutes

   

Its raining and traffic is moving slowly….it delays you by 10 minutes

   

There is a "jumper" on the bridge. Traffic is re-routed….1 hour delay

   

You hit the snooze button one extra time… ...it delays you by 10 minutes

   

A flat tire!…20 minute delay   

Your spouse forgot to set the coffee pot…it delays you by 5 minutes

   

As usual, traffic is backed up around the “5 year” bridge project…It adds 15 minutes

   

Common Cause or Special Cause?

Let’s examine the “not so simple” process of driving to work everyday

What items can be classified as

“common cause” variation and which items can we mark as special cause

Mistake Proofing(Poka-Yoke)

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Mistake-Proofing (Poka Yoke) in Everyday Life Although we often take it for granted, mistake-proofing is all around us.

• Things we use everyday have been mistake-proofed. • While many products or processes are mistake-proofed for safety

safety is not always the reason to mistake-proof.• Some things are mistake-proofed to make the product work properly,

to make it easier to use or to put together.

And mistake-proofing does not require extensive engineering efforts each time a problem is being tackled.

• Sometimes, a simple idea can be a great way to mistake-proof a process.

Sometimes, mistake-proofing means action will be initiated automatically as part of the process itself; sometimes we will get a signal alerting us to take action.

Mistake-Proofing (Poka Yoke)

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Improvement Hierarchy

Awareness

Full Automation

Fail-Proof

Simplification

SPC

Training

Des

irabi

lity

Improvement Hierarchy Levels Characteristic(s) Improvement Hierarchy Levels

No Manual Intervention

Impossible to Fail

Eliminate Activity

SPC

Certification & Audits

Notification

High chance that improvement will

have lasting results!

Less chance that improvement will

have lasting results! (Performance reverts

to old habits.)

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Mistake-proofing defined:• Using wisdom and ingenuity to create devices that

allow you to do your job 100% defect free 100% of the time.

• Poke-yoke is a quality assurance technique developed by Japanese manufacturing engineer Shigeo Shingo.

• The aim of poke-yoke is to eliminate defects in a product by preventing or correcting mistakes as early as possible.

Mistake-Proofing (Poke-Yoke)

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Mistake-Proofing (Poka-Yoke)

Office• Spell checkers in Word processing software• A “Do you want to delete?“ message

after you press the “Delete” button on your computer

Factory• Dual palm buttons and other guards

on machinery

Retail• Tamper-proof packaging

Home• Automated shutoff on electric

coffee pot • Ground fault circuit breakers for

bathroom inside or outside electric circuits

• Pilotless gas ranges and hot water heaters

• Child-proof caps on medicines• Butane lighters with a safety

button

Automobile• Seat Belts• Air bags• Car engine warning lights

Everyday examples:

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Defects vs. Errors

XY

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Mistake-Proofing States

There are two states of a defect:

ERROR ABOUT TO OCCUR

DEFECT ERROR ABOUT TO OCCUR(Prediction)

SHUTOWN(Stop Operation)

ERROR HAS OCCURED

CONTROL/FEEDBACK

WARNING SIGNAL

DEFECT HAS OCCCURRED(Detection)

SHUTOWN

CONTROL/FEEDBACK

WARNING SIGNAL

Mist

ake-

Proo

f Fun

ction

┴

┴

Poke-yoke devices fall into two major categories: prevention and detection.

• A prevention device engineers the process so that it is impossible to make a mistake at all.

• A classic example of a prevention device is the design of a 3.5 inch computer diskette.

• The diskette is carefully engineered to be slightly asymmetrical so that it will not fit into the disk drive in any orientation other than the correct one.

• Prevention devices remove the need to correct a mistake, since the user cannot make the mistake in the first place.

Categories of Poke-Yoke Devices

Categories of Poke-Yoke Devices

• A detection device signals the user when a mistake has been made, so that the user can quickly correct the problem.

• The small dish used at the Yamada Electric plant was a detection device; it alerted the worker when a spring had been forgotten.

• Detection devices typically warn the user of a problem, but they do not enforce the correction.

• We are surrounded every day by both detection and prevention poke-yoke devices, though we may not usually think of them as such.

• My microwave will not work if the door is open (a prevention device). • My car beeps if I leave the key in the ignition (a detection device). • At few years ago, some cars were designed not to start until the passengers had

buckled their seat belts (a prevention device); but this mechanism was too intrusive and was replaced by a warning beep (a detection device).

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Mistake-Proofing State Examples

Technique Prevention Detection

Shutdown

Control

Warning

Camera Low Battery

Car Oil Light

Gas Hose Nozzle Size

Clothes Dryer Lint Alarm/Shutdown

Apple Size Sorter

Smoke Detector

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Mistake-Proofing

Hints to Help:• Keep it Simple• Inexpensive• Give prompt feedback• Give prompt action (prevention)• Focused application• Have the right people’s input

Strive for the Best• BEST: Make it impossible for errors• BETTER: Allows for detection while an error is being made• GOOD: Detects a defect before it continues to the next operation

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Types of Mistake-Proofing Devices

• Guide/reference/interference rod or pin

• Template• Limit switch/micro-switch• Counter• Odd-part-out method• Sequence restriction• Standardize and solve• Critical condition indicator• Stopper/gate• Sensor

Here’s a list, certainly not all inclusive, of mistake-proofing devices that can be used to prevent errors

Floor sensors in moving shelf system prevent shelves from moving while person is in an aisle.

• Circuit breakers prevent electrical overloads and the fires that result.

• When the load becomes too great, the circuit is broken.

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Practical, Feasible, Cost-Effective:

Before implementing a solution, it is always a good ideato conduct a reality check. You should assess whetherthe solution is practical, feasible, and cost effective.

A cost-benefit analysis is one way of checking thecost-effectiveness.

A benefit of mistaking-proofing is the elimination of risk. Compare the risk of NOT implementing the solution against the cost of implementing it.

Always check with other areas you might affect!!!!

Implementing Mistake Proof Solutions

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Step 1: Use a Team Approach (a multi functional, diverse team)

Step 2: Describe the Problem (clear, complete problem statement)

Step 3: Interim Containment (Stabilize the current situation)

Step 4: Define the Root Causes (what is the underlying reason for the defect?)

Step 5: Develop (Mistake-Proofing) Solutions (Brainstorm, etc.)

Step 6: Check for Practicality, Feasibility, & Cost Effectiveness

Step 7: Implement the (Mistake-Proofing) Solution (Action Plan!)

Step 8: Prevent Recurrence (Test/Monitor the solution to make sure it works)

Step 9: Congratulate the Team (Make sure everyone celebrates!)

Implementing Mistake Proof Solutions

Pilot!

A low clearance bridge has resulted in several instances of trucks getting damaged and wedged beneath the bridge. In the latest instance, the bridge, which carries railroad tracks across the road, ripped off the top of the truck, folding the metal like an accordion.

Traffic was re-routed and crews were brought in to remove the truck. Police ticketed the driver for disobeying a traffic sign. The traffic sign warns drivers that the bridge is 11 feet above the pavement.

Mistake Proofing Exercise

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Use what you’ve learned about “error proofing” to develop a solution.

What would you do to stop trucksfrom hitting this bridge?

Mistake Proofing Exercise

Step 2: Describe the Problem (clear, complete problem statement)

Step 3: Interim Containment (Stabilize the current situation)

Step 4: Define the Root Causes (what is the underlying reason for the defect?)

Step 5: Develop (Mistake-Proofing) Solutions (Brainstorm, etc.)

Step 6: Check for Practicality, Feasibility, & Cost Effectiveness

10-15

Minutes

Mistake Proofing ExerciseWhat were your solutions??

A stop area with cross arm prior to the bridge. Photoelectric sensors tell the driver whether he/she can proceed or that he must make a U-turn.

(forces the driver to stop, won’t allow access if too high)

Raise the bridge or lower the road!! (eliminates all possible defects?)

A bar or pipe hung prior to the bridge lets the driver know if the truck is too high.

(error proof aid, but still depends on the drivers judgment)

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Variation & Mistake Proofing

Upon completing this module, students should be able to:

• Understand common cause and special cause variation

• Understand how to interpret control charts • Understand the Improvement Hierarchy• Understand the concept of Mistake Proofing • Recognize error-proofing in a process

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Project Questions

Let’s talk about your projects:• Problems you may be having• What is going well?• What is not going well?• What difficulties have you encountered?

• Any other questions for us?