mistake-proofing training john r. grout campbell school of business berry college
TRANSCRIPT
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