Mistake-Proofing Training

John R. GroutCampbell School of Business

Berry College

Each must be managed to improve

quality and reliability.

Each must be managed to improve

quality and reliability.

Non-Conformances come from many sources including:

VariationCulture Complexity Mistakes

Where does mistake-proofing fit in your quality tool box?

CultureComplexity

Variation

Mistakes

Today’s presentation:

Individually errors are rare. However, as a group they are a major cause of failures

Parts and processes can be controlled in ways that dramatically reduce the occurrence of failures due to mistakes.

Definition

Mistake The execution of a prohibited action, the failure to correctly perform a required action or the misinterpretation of information essential to the correct execution of an action

Mistake proofing the use of process or design features to

prevent manufacture of non-conforming product.

Everyday Examples

New lawn mowers are required to have a safety bar on the handle that must be pulled back in order to start the engine. If you let goof the safety bar, the mower blade stops in 3 seconds or less.

Fueling area of car has three error-proofing devices: 1. insert keeps leaded-fuel nozzle from being inserted 2. tether does not allow loss of gas cap 3. gas cap has ratchet to signal proper tightness and

prevent overtightening.

3.5 inch diskettes cannot be inserted unless diskette is oriented correctly. This is as far as a disk can be inserted upside-down. The beveled corner of the diskette along with the fact that the diskette is not square, prohibit incorrect orientation.

Why use mistake-proofing?

You can make more money

It works where other techniques don’t

evidence of effectiveness

the difficulties with human error

The final user deserves it.

Evidence of the Effectiveness

Source: Productivity Inc. and Shingo prize profiles

AT&T Power Systems is first US manufacturer to win the Deming prize. Average outgoing defects reduced by 70%

A washing machine drain pipe assembly line produced 180,000 units without a single defect (6 months).

TRW reduced customer PPM’s from 288 to 2.

Evidence of the Effectiveness

Source: Productivity Inc. and Shingo prize profiles

Cooper Automotive: 95% less defects than nearest competitor 75% less injuries 99.6% less customer defects (13 ppm) 88% in-plant defect reduction 70% less warranty cost 89% scrap reduction (0.7%) 60% productivity increase

Devices Tend to be Inexpensive...

Cost of Poka-Yoke Devices

00.10.20.30.40.50.60.70.80.9

1$2

5 o

r le

ss

$10

0 to

$250

$100

0 or

mor

e

Cost

Pro

babi

lity

Frequency of Occurrence

Cumulative Probability

$25

to $

100

$25

0 to

$10

00

Evidence of the Effectiveness

…and Very Effective

The “10:1, 100:1, even 1000:1” rates of return referred to by Bhote above are not unreasonable in practice.

Dana corporation has reported a $500,000 savings resulting from a $6 device. (83,000:1)

AT&T Power Systems (Lucent Technologies) reported net saving of $2545 per device (3300 devices) [Marchwinsky, 1997]. (25:1*)

Weber Aircraft reports saving $350,000 during their first year of implementation of approximately 300 devices. (11:1*)

*Assumes and average devise cost $100

Common Mistake-proofing Devices

Guide Pins Blinking lights and alarmsLimit switches Proximity switchesCountersChecklists

The difficulties with human error Why existing tools are not enough

Motorola findings:

...it became evident early in the project that achieving a Cp greater than 2 would go only part of the way. Mistake-proofing the design would also be required ... Mistake-proofing the design is an essential factor in achieving the [total number of defects per unit] goal.

Smith, B. IEEE Spectrum 30(9) 43-47

Errors are difficult to manage using statistics.

Pro

babi

lity

Slot width

omitted operation

normal variation

Poka-yoke & SPC

X-bar chart(measurement data)

p-chart(attribute data)

Successive-checks

orSelf-checks

use to sort outdefects, will not

improve the process

special case of np-chart,

ad hoc design, will also sort out

defects

Source-Inspection

use as correctiveaction for special

causes

use to eliminatespecific

types of defects

Error-proofing & SPC

SPC is good at detecting shifts in the process mean or variance. Changes to the process must be ongoing to be readily detected.

Human errors tend to be rare, intermittent events. They are not readily detected by control charts.

Use error-proofing (not SPC) to reduce defects caused by human error

Motorola got an order of magnitude closer to their goalusing a combination of SPC and error-proofing.

Task Type ProbabilityDetection of deviation or inspection 0.07 Alpha input per character 0.008Numeric input per character 0.003

Assembly per task element 0.00007

As a group they are commonResearch study #1 (Harris) >0.80Research study #2 (Rook) 0.82Research study #3 (Voegtlen) 0.60Research study #4 (Headlamps) >0.70NASA mishaps >0.50FAA Maintenance problems >0.94

Individually mistakes are rare

Data from “Human Reliability Data - The State of the Art and the Possibilities”Jeremy C. Williams, 1989 CEGB

To err is human

Have you ever done the following:

Driven to work and not remembered it?

Driven from work to home when you meant to stop at a store?

It happens to workers too.

Workers finish the shift and don’t remember what they have done.

After building green widgets all morning, the workers put green parts on the red widgets in the afternoon.

Corrective action

I recently polled the Quality newsgroup on the internet. A majority reported at least 20-30% of corrective actions were “worker reprimanded and retrained.”

The admonition to “be more careful” or “pay attention” are not effective for humans, especially in repetitive environments.

“Be more careful” not effective

“The old way of dealing with human error was to scold people, retrain them, and tell them to be more careful … My view is that you can’t do much to change human nature, and people are going to make mistakes. If you can’t tolerate them ... you should remove the opportunities for error.”

“Training and motivation work best when the physical part of the system is well-designed. If you train people to use poorly designed systems, they’ll be OK for awhile. Eventually, they’ll go back to what they’re used to or what’s easy, instead of what’s safe.”

“You’re not going to become world class through just training, you have to improve the system so that the easy way to do a job is also the safe, right way. The potential for human error can be dramatically reduced.”

Chappell, L. 1996. The Pokayoke Solution. Automotive News Insights, (August 5): 24i. LaBar, G. 1996. Can Ergonomics Cure ‘Human Error’? Occupational Hazards 58(4): 48-51.

A new attitude toward preventing errors:

“Think of an object’s user as attempting to do a task, getting there by imperfect approximations. Don’t think of the user as making errors; think of the actions as approximations of what is desired.”*

*Source: Norman

A New Attitude toward Preventing Errors

Make wrong actions more difficultMake it possible to reverse actions —to

“undo” them—or make it harder to do what cannot be reversed.

Make it easier to discover the errors that occur.

Make incorrect actions correct.

Judgment Inspection

Involves sorting the defects out of the acceptable product, sometimes referred to as “inspecting in quality.”

The consensus in modern quality control is that “inspecting in quality” is not an effective quality management approach.

Judgment inspection does not improve process and should be used only in the short term.

Successive checks & Self-check (post-production product inspection)

CorrectedProductAttribute

IncorrectProcess

Parameters

CorrectProductAttribute

Reworkor

ScrapSetting

Function

CorrectProcess

Parameter

CueIncorrectProductAttribute

Search &DiscoverCauses

Rework only

Source Inspection (preemptive process inspection)

SettingFunction Cue

RemedialAction

IncorrectProcess

Parameters

CorrectProcess

Parameter

CorrectProductAttribute

Source Inspection

Self-Correcting Process SettingFunction

RemedialReaction

IncorrectProcess

Parameters

CorrectProcess

Parameter

CorrectProductAttribute

RedesignedInconsequential

ProcessParameter

CorrectedProductAttribute

Error opportunity elimination (Mistake-Proof Design)

Inspection techniques

Inspection Techniques DescriptionJudgment Assesses quality of production outputs or sorts

out defects from good product.Informative Assesses process by inspecting outputs and

using information gained to control the process(a feedback loop).

Source Assesses beforehand whether the conditionsnecessary for high quality production to exist.

Setting Functions

Setting function DescriptionPhysical(Shingo’s contact),

Checks to insure the physical attributes of theproduct or process are correct and error-free.

Sequencing(Shingo’s motion step),

Checks the precedence relationship of theprocess to insure steps are conducted in thecorrect order.

Grouping or Counting(Shingo’s fixed valuemethods),

Facilitate checking that matched sets ofresources are available when needed or that thecorrect number of repetitions has occurred.

Informationenhancement(Chase and Stewart)

Determines and ensures that informationrequired in the process is available at the correcttime and place, and that it stands out against anoisy background.

Setting Functions

The real question you need to ask:

How are you going to detect an error? automatic, not dependent on human

attention fail in “detect” mode simple & low cost if possible

Regulatory Function (Cues)

RegulatoryFunction

Description

Warnings Signals mistake but allows process to continue.Gagging System shuts down. Some resetting action required to

correct the mistake.Nonresponse Action leads to no response. The process does not crash nor

does it proceed.Self-correct Process stops. System proposes correction to mistake.Let’s talk about it Process seeks information about what was really intended.Teach me Process associates correct outcomes with incorrect actions. C

ontr

ol M

etho

ds

Regulatory Function (Cues)

The real questions you need to ask:

How are you going to stop the process? the worker needs to get the message?

By audible or visible warning By prohibiting further processing

How are you going to eliminate the possibility of error? The Contrapositive of Murphy’s Law

SimplicitySymmetry

Examples

Company InspectionTechnique

SettingFunction

ControlMethod

Binney andSmith

TrinityIndustries

Mail-ordercomputer

Hierarchy of Techniques

Source inspection

Self-checks

successive-checks

Judgment inspection

Control methods

Warning methods

Informative inspectionB

e tte

rB

e tte

r

Where it works & where it does not

Both Self-Checks& Source Inspection Self-Checks only Source Inspection only

Works well

1. Manual operations& workervigilance

2. Training cost andturnover high

3. SPC ineffective

5. Attribute data6. External exceed

internal failurecost

9. Adjustment & mis-positioningpossible

10. Blamed forcustomer mistakes

11. Special causesreoccur

Does notwork well 4. Very high-speed

production7. X-bar & R charts

are effective8. Destructive tests

Put “Knowledge in the World”

provide clues about what to do: natural mappings affordances visibility feedback constraints

Precise outcomes without precise knowledge or action?

Which dial turns on the burner?

Natural Mappings:

Stove A

Stove B

How would you operate these doors?

BA C

Affordances

Push or pull? left side or right? How did you know?

visibility and feedback

Visibility means making relevant parts visible, and effectively displaying system status

Feedback means providing an immediate and obvious effect for each action taken.

Lego exercise

The contents of the bag will make a toy. Engineers and designers spent months designing, fine-tuning this product. It is intended for use by 5-10 year olds. Please try to put them together CORRECTLY. You should use all the parts.

Constraints

"The first sign of an intelligent tinkereris to save all the parts." Aldo Leopold

Here is what your bag should contain. 24 pieces total

Answer revealed here

Constraint Description ApplicationPhysical Shape and size of objects Front vs rear

control their relationship hub

Semantic Relies on clues from Face oriented meaning of the situation correctly

Cultural Adheres to known Front & rear convention lights

Logical Based on making sense of Assembly by the relationships process of

elimination

Constraints?

Source: The Design of Everyday Things, by D.A. Norman, 1988, Doubleday