humanfactors.pdf

Human Factorsand

Operator ErrorsP. L. Clemens

February 2002

2nd Edition

2

The field of Human Factors is acomplex one with many aspects. Much

has been studied. Much has beendocumented. Certainties are few,

save one:Human Operators WILL ERR!

A Few Maxims

n Operator error is inevitable.

n Human error rates are high.

n Documented rates are often wrong, often misapplied.

n Error rate controls are available, e.g.: Training / retraining / drilling

Using checklists / inspectors / backups

Using Performance Shaping Factors

Exploiting stereotypical behavior

n BUT: Person-to-person variability precludes hard rules.

3

To Err is Human

Error must be accepted as a normal component ofhuman behavior.

Humans, be they pilots, engineers or managers, willfrom time to time commit errors.

Exhortations to be professional or to be more carefulare generally ineffective, because

most errors are committed inadvertently by peoplewho are already trying to do their job professionallyand carefully. They did not intend to commit theerrors. ICARUS Committee of the Flight Safety Foundation;

Flight Safety Digest, December 1994.

4

To Err is Human

Federal Aviation Administration 2001.

iatrogenic injury contributes to 180,000 deathsannually in the United States.

Annals of Internal Medicine; Vol. 132, No. 9; 2 May 2000.

More than seventy percent of all crashes of scheduledcommercial aircraft are caused directly by controlled

flight into terrain.

I dont want to make the wrong mistake!Yogi Berra

5

Classes of Human Error

n Errors of Commission: Performing correct step on wrong item.

Performing step incorrectly on right item.

Performing correct step at wrong time.

n Errors of Omission: Skipping a necessary step.

Failing to communicate to a fellow worker (spoken or written).

n Cognitive Task Errors: Forming an incorrect diagnosis (faulty data processing, flawed

logic, improperly interpreted input information).

Making a decision unsupported by available information(leaping to conclusion; reliance on flawed intuitive skills).

CAUTION:If a step isntthought of, it

isntanalyzed!

SABOTAGEis difficult to

analyze!

6

* Error of omission/item embedded in procedure.. 3 x 103

* Simple arithmetic error with self-checking.......3 x 102

* Inspector oversight of operator error...................101

* General rate / high stress/dangerous activity.. 0.2 - 0.3** Checkoff provision improperly used................ 0.1 - 0.9 (0.5 avg.)** Error of omission / 10-Item check list...........104 - 5 x 103

(1 x 103 avg.)** Carry out plant policy / no check on operator.. 5 x 103 - 5 x 102

(5 x 102 avg.)** Select wrong control / group of

identical, labeled, controls.................103 - 102 (3 x 103 ave.)Sources: * WASH-1400 (NUREG-75/014); Reactor Safety Study An Assessment of Accident

Risks in U.S. Commercial Nuclear Power Plants, 1975(See Appendix I, attached.)

** NUREG/CR-1278; Handbook of Human Reliability Analysis with Emphasis onNuclear Power Plant Applications, 1980

Note: Similar data appear for similar activities in Loss Prevention in the Process Industries;2nd Edition; F. P. Lees; Butterworths; 1996; ISBN 0 7506 1547 8.

ACTIVITY ERROR RATE

Typical Human Operator Failure Rates

For additionaldata, see

Appendix 2.

7

Societal Expectations Vary Widely

n Esteem for a worker with a 66.6% error rate.(BHF example achieving a 0.333 LBA givesHall of Fame entry.)

n Termination of a worker for a single error.(Transit system operator example One strikeand youre out!)

Typical extremes

8

Mismatches Aboundbetween analytical predictions

and experience:n WASH-1400 Human error probability for routine

repetitive tasks: 3 x 103 to 1 x 102 per individual operation.*

n Concert pianist, performing K.453, 1st movement:3996 individual, critical keystrokes.**

n Expected errors per performance: 3996 x (3 x 103) 12 to 3996 x (1 x 102) 40 errors

A DOOMED REPUTATION!

SOURCES: * WASH-1400 (NUREG-75/014); Reactor Safety Study An Assessment ofAccident Risks in U.S. Commercial Nuclear Power Plants, 1975.

** Mozart, W. A.; 17th Piano Concerto (incl. Composers own first cadenza).

9

Many Factors InfluenceHuman Operator Failure Probability

n Experience

n Stress

n Training

n Fatigue

n Level of burden

n Work station design (e.g., priority placement of critical controls)

n Conscientiousness (self discipline)

n Realization of error on prior attempt

n Perception of error consequences (to self / others)

n Character of task (complexity / repetitiveness)

n Environment (temperature / noise / comfort)

n Task violation of stereotypical behavior

n many others

Population group norms are intuitively predictable.For some individuals, the influence is counterintuitive!We dont hire population groups. We hire individuals!

FOR CRITICAL TASKS, KNOW YOUR INDIVIDUALS!

Some factors are PHYSIOLOGICAL.Some factors are PSYCHOLOGICAL.

Some are BOTH.

Examples:

10

Influence of Stress*

Panic!

Stress

Fai

lure

Pro

bab

ility

VeryLow

ExtremelyHigh

ModeratelyHigh

1.0

*Based on: Human Error Evaluation and Human Reliability Analysis; B. J. Bell; American Institute of Chemical Engineers

Optimum

11

Controlling Operator Error Probability PERFORMANCE SHAPING FACTORS: Use procedures. Use checklists. Make systems forgiving. Provide training / retraining (talkthroughs /

walkthroughs, simulations, drills, verificationexaminations, certification, etc.). Use backup inspectors, observer feedback. Adjust operator burden. Use pacing (fatigue avoidance). Provide revokability. (Design for time / opportunity /

means to recognize and to correct errors.)Place critical operations under automatic control, i.e.,

AUTOMATE, AUTOMATE, AUTOMATE!

12

AUTOMATE, AUTOMATE, AUTOMATE?

ButWhen should

weNOT

AUTOMATE?

Use a human operator when a component isneeded that can think on its feet, processinginformation in ways the need for which could

not have been appreciated prior to themoment of need, and that can then act

effectively using the result!

13

More Performance Shaping Factors

n Physiological Speed / Strength demands Task Design Tension / Fatigue Task Design Environment: System Design

Temperature; Humidity; Noise

n Psychological Emotional State Train / Drill Mental Stress Train / Drill Preparedness (actual) Train / Drill Preparedness (sensed) Train / Drill Realization of prior success / failure Train / Drill Personality / attitude Worker Selection Stereotypical response Worker Selection

affects one ormore of these.

Each ofthese

MITIGATORS

14

Influence of Training and Drills*

*Based on: Human Error Evaluation and Human Reliability Analysis; B. J. Bell; American Institute of Chemical Engineers

Time

Res

po

nse

Pro

fici

ency

Initial TrainingDrilling, Testing,

Certification

Retraining,Drilling, Testing

WithRetraining,

DrillingWithout

Retraining,Drilling

15

Stereotyping

Where do youShower?

COUNTRY

USA-------------------H

France--------------- C

Italy------------------- C

Spain-----------------C

Canada (West)---- H

Canada (East)----- C

Mexico---------------C

HotWaterFaucetLabel

Which way do yourotate the control for

increased effect?

GardenHose

Spigot

RadioVolumeControl

When designing system controls and operating procedures

Do not defy stereotypes!

Do take advantage of stereotypes!

TEST YOUR STEREOTYPING: Cross your arms. Which hand is on top? Can you reverse it?

16

Analytical Methods

n A brief look at four approaches:

Failure Modes and Effects Analysis

Technique for Human Error Rate Prediction(an Event Tree Analysis adaptation)

Fault Tree Analysis

Management Oversight and Risk Tree Analysis

17

n Each Task has many Steps. Each Step has many Failure Modes. Any single Operator Action may be

performed: Incorrectly (many possible modes) Too early Too late Too long Too briefly Not at all (omission) et alia

n Variations: Correct Operation to Wrong Item Wrong Operation to Correct Item Wrong Operation to Wrong Item et alia

*Failure Modes and Effects Analysis

CAUTION:FMEA ONLY

Identifies potentialSingle Point errors

with assurance!

CAUTION:If a step isnt

thought of, it isntanalyzed!

The FMEA* Approach

FMEA1) Identify individual Task Steps.

2) Identify Failure Modes foreach Task Step.

3) Evaluate Effects of eachFailure Mode on each Target.

18

An FMEA Example

1. Open receptacle Circuit Breaker.

2. Remove Receptacle Wall Plate (two screws).

3. Remove Old Receptacle (two screws).

4. Unwire Old Receptacle.

5. Wire New Receptacle:

1. Green wire to large brass screw.

2. White wire to silver screw.

3. Black wire to copper screw.

6. Install New Receptacle (two screws).

7. Replace Wall Plate (two screws).

8. Close Circuit Breaker.

9. Test New Wall Receptacle (Table Lamp).

Replacing a Household Electrical Receptacle

Identify individual Task Steps:



Lockout-Tagout?

PROCEDURE

19

An FMEA Example

Step FailureMode

FailureEffect

Severity Prob-ability

RiskCode

1) Open CircuitBreaker

2) RemoveReceptacleWall Plate

Select wrongBreaker

Out ofsequence/early

Kill power tocritical item

Fail to protectduring receptaclereplacement /shock

II(Equipment)

CSelect wrongBreaker

Fail to protectduring receptaclereplacement /shock

I(Personnel)

I(Personnel)

C

D

1

1

2



20

THERP* Method (Event Tree Format)

TheoriginalSwain

THERPformat.

TaskAttempted

TaskPerformedCorrectly

TaskPerformedIncorrectly

ErrorDiscovered(Inspection)

ErrorUndiscovered

ErrorCorrection

Not Attempted

CorrectionSucceeds

CorrectionFails

Success

Failure

Success

Failure

Failure

ErrorCorrectionAttempted

*Technique for Human Error Rate Prediction

For one taskstep, among

many!

21

The Fault Tree Method

TASKPERFORMANCE

FAILURE

InitialPer-

formanceFails

Inspection /Correction

Fails

CorrectionFails

ErrorUndis-

covered

CorrectionNot

Attempted

For one taskstep, among

many!

Weve assumed: Need for the Task has been

recognized, not ignored.

The Task has beenattempted, not avoided.

22

Fault Trees find Man Paths to TOPTOP

1V

2m

6h

GE

D

CB

A

5h

PT

1v

3h

4h3h 4h

F

Some initiators may be vulnerable to severalCommon Causes and receive several correspondingsubscript designators. Some may have no CommonCause vulnerability receive no subscripts.

Uniquely subscriptinitiators, using letterindicators of suscept-ibility, e.g.

m = moistureh = human operatorq = heatf = coldv = vibrationl = locationetc.

Find Minimal Cut Sets

All initiators in this CutSet are vulnerable tohuman operator error.Operator error alone caninduce TOP.

1V 2m

1v 3h

1v 4h

3h 4h 5h 6h

Cut Set Probability (Pk), the product ofprobabilities for events within the CutSet, is the probability that the Cut Setbeing considered will induce TOP.

Pk = Pe = P1 x P2 x P3Pn

23

LOSSEVENT

!

LTA LTA LTA

LTA LTA LTA

LTA LTA LTA

LTA LTA LTA

The MORT* View of Operator Error

*Management Oversight and Risk Tree analysis

TrainingCriteriaMethodsPrior SkillVerification (Testing /

Certification) Motivation

Schedule PressureJob InterestConflicts w/OthersDeviant Behavior

Physical FactorsDisplaysControls

many others

LTA (Less Than Adequate)Contributors:

TrainingCriteriaMethodsPrior SkillVerification (Testing /Certification)

MotivationSchedule PressureJob InterestConflicts w/OthersDeviant Behavior

Physical FactorsDisplaysControls

many others

24

The Analysis Challenge Hardware Equipment

A tractable population ofcomponents, with relatively

few, well defined, failuremodes amenable to analysis.

V-15NO

RECIRC.

P-4

V-16

V-17NO

V-20NO

V-18NC

V-21

V-22

PROC. C

PROC. D

HH-4

V-19NO

V-23

P-3

25

Opqstvi sprtuActuiniumalFohixinrwo























Folpie Smartsies100

50

0

Folpie Smartsies100

50

0






























The Analysis Challenge the Human Operator

Many steps in manytasks, each with manymarginally ponderable

failure modes.

26

Reminder Principles

n Human Operators err!

n To reduce error probability: Make systems forgiving give the operator

fault annunciation, time, opportunity to revokeerrors.

Use documented procedures / checklists. Exploit stereotype behavior. Adjust operator burden apply pacing. Use backups, inspectors / checkers. Train, examine, simulate, rehearse, drill, retrain. AUTOMATE, AUTOMATE, AUTOMATE!

Analyses treat only the things we look for!

27

Example Human Error Rates

Note 1. Modification of these underlying (basic) probabilities were [sic] made on the basis ofindividual factors pertaining to the tasks evaluated.

Note 2. Unless otherwise indicated, estimates of error rates assume no undue time pressuresor stresses related to accidents.

Appendix 1

From: WASH-1400 (NUREG-75/014); Reactor Safety Study An Assessment ofAccident Risks in U.S. Commercial Nuclear Power Plants, Appendix III 1975.

EstimatedRates (x) Activity

1 x 104/D Selection of a key-operated switch rather than a non-key switch (this value does not include theerror of decision where the operator misinterprets situation and believes key switch is correctchoice).

1 x 103/D Selection of a switch (or pair of switches) dissimilar in shape or location to the desired switch(or pair of switches), assuming no decision error. For example, operator actuates large handleswitch rather than small switch.

3 x 103/D General human error of commission, e.g., misreading label and therefore selecting wrongswitch.

28


1 x 102/D General human error of omission when there is no display in the control room of the status ofthe item omitted, e.g., failure to return manually operated test valve to proper configuration aftermaintenance.

3 x 103/D Errors of omission, where the items being omitted are embedded in a procedure rather than atthe end as above.

3 x 102/D Simple arithmetic errors with self-checking but without repeating the calculation by redoing it onanother piece of paper.

1/x Given that an operator is reaching for an incorrect switch (or pair of switches), he selects aparticular similar appearing switch (or pair of switches), where x = the number of incorrectswitches (or pair of switches) adjacent to the desired switch (or pair of switches). The 1/xapplies up to 5 or 6 items. After that point the error rate would be lower because the operatorwould take more time to search. With up to 5 or 6 items he doesnt expect to be wrong andtherefore is more likely to do less deliberate searching.

1 x 101/D Given that an operator is reaching for a wrong motor operated valve (MOV) switch (or pair of switches), he fails to note from the indicator lamps that the MOV(s) is (are) already in thedesired state and merely changes the status of the MOV(s) without recognizing he hadselected the wrong switch(es).

1.0/D Same as above, except that the state(s) of the incorrect switch(es) is (are) not the desiredstate.

Appendix 1


29


Appendix 1


1.0/D If an operator fails to operate correctly one of two closely coupled valves or switches in aprocedural step, he also fails to correctly operate the other valve.

1 x 101/D Monitor or inspector fails to recognize initial error by operator. Note: With continuing feedbackof the error on the annunciator panel, this high error rate would not apply.

1 x 101/D Personnel on different work shift fail to check condition of hardware unless required by checklist or written directive.

5 x 101/D Monitor fails to detect undesired position of valves, etc., during general walkaroundinspections, assuming no check list is used.

2 - 3 x General error rate given very high stress levels where dangerous activities are occurring101/D rapidly.

2(n1)x Given severe time stress, as in trying to compensate for an error made in an emergencysituation, the initial error rate, x, for an activity doubles for each attempt, n, after a previousincorrect attempt, until the limiting condition of an error rate of 1.0 is reached or until time runsout. This limiting condition corresponds to an individuals becoming completely disorganized orineffective.

1.0/D Operator fails to act correctly in the first 60 seconds after the onset of an extremely high stresscondition, e.g., a large LOCA.

30


Appendix 1


9 x 101/D Operator fails to act correctly after the first 5 minutes after the onset of an extremely high stresscondition.

1 x 101/D Operator fails to act correctly after the first 30 minutes in an extreme stress condition.

1 x 102/D Operator fails to act correctly after the first several hours in a high stress condition.

x After 7 days after a large LOCA, there is a complete recovery to the normal error rate, x, for anytask.

31

Appendix 2

Bibliography

Human Engineering Design Criteria for Military Systems, Equipment and Facilities MIL-STD-1472D. A frequently cited military contract design criteria document related to the practice of humanengineering. Covered are visual displays, audio displays, controls, labeling, anthropometry, workspacedesign, and environment. 1989 Soft cover; large format; 388 pp.

Human Factors Engineering Design for Army Materiel (Metric) MIL-HDBK-759A. A handbook ofhuman factors engineering design data and detailed criteria, assembled as a reference to provide guidanceto designers of Army materiel, and broadly applicable to many other fields as well. 1981 Soft cover;large format; 677 pp.

A Procedure for Conducting a Human Reliability Analysis for Nuclear Power Plants NUREG/ CR-2254 B. J. Bell and A. D. Swain. A detailed procedure is presented, to be followed in conducting a humanreliability analysis as part of a probabilistic risk assessment. An overview of the procedure describes themajor elements of a human reliability analysis, along with a detailed description of each element. Anexample of an actual analysis is presented. 1983 Government Printing Office; Soft cover; large format;120 pp.

Human Factors Design Guidelines for Maintainability of Department of Energy Nuclear Facilities UCRL-15673 / PSA 4312705 James P. Bongarra, Jr. et al. Maintainability design concepts are presentedwhich are applicable in a broad range of industrial settings. Many practical examples are given and aregraphically illustrated. 1985 Lawrence Livermore National Laboratory; Soft cover; large format; 161pp.

Human Factors and Operator ErrorsA Few MaximsTo Err is HumanClasses of Human ErrorTypical Human Operator Failure RatesSocietal Expectations Vary WidelyMismatches AboundMany Factors Influence Human Operator Failure ProbabilityInfluence of Stress*Controlling Operator Error ProbabilityAUTOMATE, AUTOMATE, AUTOMATE?More Performance Shaping FactorsInfluence of Training and Drills*StereotypingAnalytical MethodsThe FMEA* ApproachAn FMEA ExampleTHERP* Method (Event Tree Format)The Fault Tree MethodFault Trees find Man Paths to TOPThe MORT* View of Operator ErrorThe Analysis Challenge Hardware Equipment The Analysis Challenge the Human OperatorReminder PrinciplesExample Human Error RatesBibliography

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