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Human ReliabilityAssessment
Grace Kennedy
16th/19thOctober 200606ELD061/06ELP461
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Objectives for the Sessions
Understand the Human Reliability Assessment process Gain practical experience of a simple task analysis
Gain practical experience of error identification
Gain practical experience of representation
Gain practical experience of quantifying errorprobabilities in a simple example
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Human Error Example 1
A KC-135 Aircraft was being pressurised at groundlevel.
The outflow valves were capped off during a 5 yearoverhaul and never re-opened.
A civilian depot technician was using a home-madegauge, and no procedure.
The technician's gauge didn't have a max "peg" forthe needle which had gone round the gauge more
than once.
The result.
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Human Error Example
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Human Error Example 2
Helios Crash 2005
Extract taken from BBC News Site
http://news.bbc.co.uk/1/hi/world/europe/6036507.stm?ls
Pilot misread instruments ANDmisinterpreted warning signals
Maintenance left pressure control
in wrong settingManufacturer did not respondadequately to previous similarincidents
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Predicting errors
Task analysis and error identification
Preventing errors
Specifying training requirements
Equipment design (e.g. pressure gauge)
Detailed procedures (administrative control)
Ultimately: Reduce risk
Save money
Justify design decisions
What can be done about it?
For every $1 spent in theearly stage,
approximately $10,000 aresaved (if the problemwere to be fixed laterinstead). Manprint
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HRA Process Outline
Task analysis is used to describe and understand the
human interactions with the system The results of the task analysis are used with an error
taxonomy (classification scheme) to allow erroridentification
The identified errors are analysed either qualitativelyor quantitatively
The process is repeated each time a design iterationoccurs
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Human Reliability Assessment Process
General HRA Process Ki rwan , 1994
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Human Reliability Assessment Process
Problem Definition Task Analysis
Describe what is done
Improve analysts knowledge
Error Identification
Taxonomy
Failure criteria
Representation
Fault tree/event tree
Risk model
Quantification
e.g. HEART
Impact Assessment Effect of errors
Risk contribution
Error Reduction
Re-design tasks
Add engineered features Procedures / training
Quality Assurance
Appropriate techniques
Technical checking
Documentation
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HRA Techniques
Many HRA techniques available
Working to different levels of detail ondifferent concepts
From expert judgement techniques (e.g. APJ,
PC) Hazard identification techniques (e.g.
HAZOPS, THEA)
To quantitative techniques (e.g. HEART,
THERP)
To second generation techniques (e.g. CREAM,ATHEANA)
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Human Reliability Assessment Process
General HRA Process Ki rwan , 1994
TASK ANALYSIS
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Task Analysis
Range of techniques to understand what humans arerequired to do in order to achieve a system goal
Collect and organise information
Improve the analysts understanding
Structured approach
Support to design and assessment
A Guide to Task Analysis,
Barry Kirwan & Les Ainsworth (1992),
Taylor and Francis
ISBN 07484-0058-3
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Hierarchical Task Analysis
Expresses a job or function in terms of goals,
operations and plans
Goals Objectives to be achieved
Operations Actions required to achieve the goals
Plans Conditions under which the actions are
carried out
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Hierarchical Task Analysis Example
Express the task of making a cup of tea using HTA
Goals Objectives to be achieved (e.g. Make Tea)
Operations Actions required to achieve the goals (e.g.
Boil water, Add milk / sugar)
Plans Conditions under which the actions are
carried out (e.g. boil the water before adding it
to the cup)
Example provided using TaskArchitect software
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Making Tea - one solution (1 of 5)
Bar beneath
the activity
shows no
further
development
Stub beneath
the activity
shows further
development
has takenplace
Plan describes
the logic
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Making Tea - one solution (2 of 5)
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Making Tea - one solution (3 of 5)
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Making Tea - one solution (4 of 5)
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Hierarchical Task Analysis - Practical
Express the task of fitting an electric plug using HTA
Goals Objectives to be achieved (e.g. Fit plug)
Operations Actions required to achieve the goals (e.g.
Strip outer casing, Twist exposed wires)
Plans Conditions under which the actions are
carried out (e.g. fit the fuse before closing up
the plug to the cup)
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Hierarchical Task Analysis - Guidance
State an overall goal (box at the top)
Breakdown each goal or sub-goal one at a time (i.e.
finish one box before moving on)
Ensure all the actions under a goal are relevant and
would actually achieve the stated goal
Keep the order and logic in the plans (make the plans
specific to the goal)
Work in Pairs - 10 m inutes A solution on pink sheet
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Wiring a plugHTA one solution (1 of 4)
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Wiring a plugHTA one solution (2 of 4)
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Wiring a plugHTA one solution (3 of 4)
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Wiring a plugHTA one solution (4 of 4)
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Human Reliability Assessment Process
General HRA ProcessKi rwan , 1994
ERROR
IDENTIFICATION
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Error Identification - General
Task Analysisdescribes the activities necessary toachieve a goal
An Error Taxonomy(classification scheme) can beused to identify specific errors
Many errors will be possible, so need to understand Error effects (relating to the task goal)
Failure criteria (goal failure)
Produce a list of identified errors, which lead to goal
failure
Organise the information in a Tabular Task Analysis
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Error Identification - Tabular Task Analysis
Use the information from the HTA Create a Tabular Task Analysis(TTA)
Error taxonomy (classification scheme) to identifyerrors
Understand
Error effects
Failure criteria
List of identified errors
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Tabular Listing from HTAID Task Plan
0 Wire an electric plug do in sequence 1-5
0.1 Collect tools
0.2 Unscrew plug cover
0.3 Prepare lead do in sequence 1-60.3.1 Estimate length of stripped wire required to reach earth terminal
0.3.2 Strip outer casing according to estimate
0.3.3
Check yellow/green wire reaches earth terminal whilst outer casing
exceeds holder by 5 mm
0.3.4 Cut blue and brown wires to reach their terminals
0.3.5 Strip each of the coloured leads to leave exposed wire
0.3.6 Twist exposed wires on each coloured lead
0.4 Ensure correct fuse is in place
do in sequence 1-3; If mismatch
between required and in-situfuse then do ( 4)
0.4.1 Locate appropriate instructions for equipment
0.4.2 Read fuse requirement
0.4.3 Compare fuse requirement with given fuse
0.4.4 Change the fuse do in sequence 1-3
0.4.4.1 Select the correct fuse
0.4.4.2 Extract fuse from plug
0.4.4.3 Insert correct fuse
0.5 Attach plug to lead do in sequence 1-4
0.5.1 Thread lead through holder
0.5.2 Fit each twisted wire to correct terminal
0.5.2.1 Select one coloured lead
0.5.2.2 Identify lead based on colour (earth, live, neutral)
0.5.2.3 Locate correct terminal for selected lead
0.5.2.4 Unscrew terminal
0.5.2.5 Place exposed twisted wire through terminal hole
0.5.2.6 Tighten terminal screw
0.5.3 Secure main lead holder
0.5.4 Replace plug cover
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TTA ExampleSelected ActivitiesID Task Plan Error
ID
Error Type Immediate
effects of error
Detection of
error
Recovery of
error
0.4 Ensure correct fuse is in place do in sequence
1-3; If mismatch
between
required and in-
situ fuse then
do ( 4)
0.4.1 Locate appropriate instructions for
equipment
0.4.2 Read fuse requirement
0.4.3 Compare fuse requirement with
given fuse
0.4.4 Change the fuse do in sequence
1-3
0.4.4.1 Select the correct fuse
0.4.4.2 Extract fuse from plug
0.4.4.3 Insert correct fuse
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Error Taxonomy
Classification scheme
Generic error types
Similar to HAZOP guidewords
Taxonomy can be made domain specific
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Error TaxonomySHERPA (see handout)
Example error types for an action task
E3 Action Omitted
E4 Action too much
E5 Action too little
E9 Right action wrong object
E10 Wrong action right object
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Tabular Task Analysis Example - PracticalID Task Plan Error
ID
Error Type Immediate
effects of error
Detection of
error
Recovery of
error
0.4 Ensure correct fuse is in place do in sequence
1-3; If mismatchbetween
0.4.1 Locate appropriate instructions for
equipment
E3 Act ion omit ted Instruct ions not
obtained
Unable to
confirm fuse
type
Re-start task
with
instructions
E16 Wrong
information
obtained
Instructions for
another device
obtained
May not
detect
Re-start task
with
instructions
0.4.2 Read fuse requirement
0.4.3 Compare fuse requirement with
given fuse
0.4.4 Change the fuse do in sequence
1-3
0.4.4.1 Select the correct fuse
0.4.4.2 Extract fuse from plug
0.4.4.3 Insert correct fuse
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Tabular Task Analysis - Guidance
See Handout (blank TTA)
Review each activity one at a time
Read through the generic errors in SHERPA
Add error types to the TTA and fill-in the remaining
columns (see example for guidance)
Work in Group s (max. 5) - 10 m inu tes
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Tabular Task AnalysisSolution to Practical
See handout sheet for example error types againsteach activity (example is not a comprehensive record)
ID Task Plan Error
ID
Er ro r T yp e I mmed ia te
effects of error
Detection of
error
Recovery of
error
0. 4 E ns ure c orrec t
fuse is in
place
do in sequence
1-3; If mismatch
between
required and in-
situ fuse then
do ( 4)
0.4.1 Locateappropriate
instructions for
E3 Action omitted Instructions notobtained
Unable toconfirm fuse
type
Re-start taskwith
instructions
E 16 W rong
information
obtained
Instructions for
another device
obtained
May not
detect
Re-start task
with
instructions
0. 4. 2 Read fus e
requirement
E3 Act ion omi tted F use type no t
obtained
Unable to
complete
step 0.4.3
Return to
instructions
E 16 W rong
information
obtained
Incorrect fuse
information used
May not
detect -
device could
fail
Repeat task
with correct
information
0. 4. 3 Compare f us e
requirement
with given fuse
E11 Check omitted Incorrect in-situ
fuse not detected
May not
detect -
device could
fail
Repeat t ask
E 16 W rong
information
obtained
Incorrect in-situ
fuse not detected
May not
detect -
device could
fail
Repeat t ask
0. 4. 4 Change t he
fuse
do in sequence
1-3
0 .4 .4 .1 Selec t the
correct fuse
E3 Action omitted No replacement
fuse
Unable to
complete
step 0.4.4.3
Repeat t ask
E9 Right action on
wrong object
Incorrect fuse
selected
May not
detect -
device could
fail
Repeat t ask
0.4.4.2 Extract fuse
from plug
E3 Action omitted Original fuse left in
place
Unable to
complete
step 0.4.4.3
Repeat t ask
0.4.4.3 Insert correct
fuse
E3 Act ion omi tted No fuse fi tted Devi ce does
not work
Repeat t ask
E17 Misalign Fuse fitted
incorrectly
Device does
not work
Repeat t ask
Example TTA - Note this TTA is not comprehensive as additional error types may apply
You cannot read this, but . . .
see green handout
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Error Identification - Question
Assume the fuse must be changed Review the tasks to achieve the goal at 0.4
Use the error taxonomy (SHERPA) to identify :
Example of an error leading to no fuse being fitted
Example of an error leading to the incorrect fusebeing fitted
Remember
Error Effects Failure Criteria
Work in pairs - 5 m inu tes
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Error Identification Question - One Solution
Errors leading to no fuse being fitted Step 0.4.4.3, Error E3 Action omitted
Errors leading to the incorrect fuse being fitted
Step 0.4.3, Error E11 Check omitted
Step 0.4.4.1, Error E9 Right action on wrong object
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Human Reliability Assessment Process
General HRA ProcessKi rwan , 1994
Representation
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Human Reliability Assessment and Risk Models
Risk models will usually include human errors forquantification (human as mitigation)
Human Reliability Assessor will collaborate with the
Risk Modeller
Further investigation may be needed in order to carry out
Human Reliability Assessment
Additional errors may be identified for inclusion in the risk
model
Changes to models may be necessary to represent human
error
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Risk Assessment - General
Risk = Frequency x Consequence/Severity Assessment of a complex system requires a structured
process (Probabilistic Safety Assessment)
Operation of the system is represented by a model (risk
model) Risk model represents features in the system that
prevent or mitigate against serious consequences (e.g.safety systems, intervention from human operators)
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Risk ModelsA Whirlwind Tour
Hazard identification process used to establish a set ofinitiating events (what can happen to the system)
Frequency of each initiator is assessed
Consider the effects of each initiator on the system
Typically use event trees to model accident sequences
System features are modelled as events in an EventTree (ask success/failure questions as top events)
Fault trees used to investigate detailed causes ofequipment/system/human failure
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An Event Tree
S
F
Respond
to Alarm
Shut
Valve
Start
Pump
Success
Success (recovered)
Failure 1
Failure 2
Failure probability = x Success probability = 1 -x
x
1 - xInitiating
event
system
leak
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An Event Tree - Quantified
S
F
Respond
to Alarm
Shut
Valve
Start
Pump
S1
S2
F1
F2
P(F) = F1 + F2 = (0.999 x 0.01 x 0.01) + 0.001 = 0.0011
P(S) = 1Failure = 10.0011 = 0.9989
0.001
0.999Initiating
event
system
leak
0.01
0.01
0.99
0.99
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A Fault Tree
Valve Fails
to Shut
Electrical
signal tovalve fails
Mechanical
valve failure
Operator fails to
demand valve toshut
OR
A B C
Failure probability = A + B + CABACBC + ABC
A
CB
For ORuse A UB UC
For ANDuse A nB nC
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Fault Tree - Practical Example
Create a Fault Tree for incorrect fuse in place (i.e. 0.4)
Two types of boolean operators
1. OR
Occurrence of ANY event below causes failure above
2. AND
Only the occurrence of ALL events below causes failure above
OR
AND
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Fault Tree - Practical Example Guidance
Use the errors identified as the branches to the trees Think about the HTA to give an indication of the
layers required
Think about which operator to use
Work in Group s (max. 5) - 10 m inu tesA solution on blue sheet
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Fault Tree - A Solution
Incorrect fuse is in place
Appropriate
instructions not
found
Fuse requirements
not read
Fuse requirement
not compared with
given fuse
Fuse not changed
Correct fuse not
selected
Fuse not extracted
from plug
Correct fuse not
inserted
OR
OR
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Human Reliability Assessment Process
General HRA ProcessKi rwan , 1994
QUANTIFICATION
Q f
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Human Error Probabilities - Quantification
Human Reliability Assessment exists to providequantification of the probability of human error
Human Error Probabilities are used in ProbabilisticSafety Assessment (risk models)
Obtaining or generating Human Error Probabilitiesrequires a range of techniques
W t t Q tit ti D t
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Ways to get Quantitative Data
Historical records
Collected data (direct or simulated)
Estimation techniques (constructive, comparative)
Judgement and experience
Hi t i l R d / C ll t d D t
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Historical Records / Collected Data
Number of recorded events of interest over time providesfrequency of error
Number of recorded events of interest over number of chances forevent to occur provides the probability of error
Strengths
specific to the error of interest
data validity (true values)
Weaknesses
may not have recorded all instances of error (under estimate)
may need a lot of data to get a fair answer
hard to identify root of some errors
collection method may affect reliability
collection in simulators may not be realistic for actual errors
design changes over time may affect reliability
A S l ti f E ti ti T h i
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A Selection of Estimation Techniques
Technique for Human Error Rate Prediction (THERP) Human Error Assessment and Reduction Technique
(HEART)
Success Likelihood Index Methodology (SLIM)
Paired Comparisons (PC)
E ti ti THERP (1)
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EstimationTHERP (1)
Technique for Human Error Rate Prediction(NUREG/CR-1278, 1983)
Collected data from civil PWRs in the USA (mainlycontrol room actions, some manual valve actions)
Presented as a database of Human Error Probabilities
Flowchart of options to navigate the database
Simple error taxonomy (omission, commission)
Includes human error dependence model Construct HEPs from basic error data
E ti ti THERP (2)
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Estimation - THERP (2)
Strengths
Powerful method with good auditability andsupporting qualitative material
Well suited to proceduralised, structured
assessments Weaknesses
Resource intensive (better with more experience)
Not adaptive Limited error reduction information
E ti ti HEART (1)
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Estimation - HEART (1)
Human Error Assessment and Reduction Technique(Williams, 1990)
Generic Task (GT) data in 9 categories
List of 38 Error Producing Conditions (EPC)
Select GT based on descriptions and examples (eachtask has a reliability attached)
Modify base reliability by considering EPCs
Advice included on possible error reduction measures
E ti ti HEART (2)
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Estimation - HEART (2)
Strengths
Simple method, not resource intensive
Error reduction suggestions
Versatile (generic nature adapts to many tasks)
Weaknesses
Does not model dependence
Results can vary greatly dependent on initial assessment (GTselection)
HEARTPC Demohttp://www.ewe.ch/regional/regional_2_6.html
Estimation SLIM (1)
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Estimation - SLIM (1)
Success Likelihood Index Method (Embrey et al, 1984)
Panel of assessors (including subject experts) Evaluate performance shaping factors (PSFs) for task of interest
Assign weighting as to the relative importance of the PSF to eachother. Assign rating based on how useful the PSF is for the task.
SLI derived from the sum of the ratings and weightings (ranks theerrors)
Calibrate SLIs with known data to convert SLI to HEP
P4P3P1 P2
Known data points
SLI
Estimation SLIM (2)
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Estimation - SLIM (2)
Strengths
Flexible technique, good theoretical method
Does not need task decomposition (task analysisand error taxonomies)
Weaknesses
Complex method, resource intensive
Lack of valid calibration data (known values)
Estimation Paired Comparisons (1)
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Estimation - Paired Comparisons (1)
Paired comparisons (Hunns, 1982)
Panel of assessors (including subject experts)
Each assessor compares all possible pairs of errordescriptions (decide which of the two is more likely foreach pair)
Combine all comparisons made by all assessors toproduce a relative scaling of error likelihood
Calibrate scaling with known data to convert to HEPs
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Judgement / Experience
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Judgement / Experience
Expert judgement
Judgement/Experience APJ (1)
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Judgement/ExperienceAPJ (1)
Absolute Probability Judgement
Panel of experts to provide direct generation of HEPs(subject experts and HRA expert)
Assumes assessors are capable of making suchestimates of reliability
Describe the tasks of interest
Describe errors and estimate HEPs
Individual estimates aggregated Group consensus of estimates
Judgement / Experience APJ (2)
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Judgement / Experience - APJ (2)
Strengths
Simple method, allows constructive qualitativediscussion
Practical error reduction measures can bediscussed during the assessment
Weaknesses
Prone to biases, may have little face validity
Needs experienced experts
Criteria for Quantification Technique Selection
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Criteria for Quantification Technique Selection
Availability of data Applicability of data
Ease of use (time, cost, resources, information)
Data validity (justification)
Experience of assessor
Level of assessment needed
HEART Practical Example
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HEART - Practical Example
Use HEART to estimate the probability of fitting an
electric plug to a device incorrectly
Assume :
Time shortage for the task Written procedure (
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HEARTSee Handout
HEART paper
Generic task descriptions
Failure probabilities for each task description
(50th percentile value should be used, 5thand 95thpercentiles
indicate the uncertainty/range)
Error producing conditions and associated
multiplication factors
HEART - Practical Example Guidance
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HEART - Practical Example Guidance
Read the Generic Task Descriptions Consider the task complexity and difficulty by
examining the identified errors from the TTA
Select a Generic Task
Review the EPCs and select the ones you believe arerelevant
Modify the Generic Task base HEP using factors forselected EPCs
See the worked example in the HEART paper
EPC equation developed to avoid negative probabilities
Work in Group s (max. 5) - 10 m inu tesA solution on yellow sheet
HEART Practical - One (simple) Solution
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HEART Practical - One (simple) Solution
Probability of fitting an electric plug to a deviceincorrectly
Generic Task F 0.003
EPC 2 time shortage, x 11
HEP = (0.003)(11) = 3.3E-2
Quantification Summary
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Quantification Summary
A range of HRA techniques is available
Technique selection depends on the nature of the assessment
Human Reliability Data can be difficult to obtain
Human Reliability Data can be uncertain (range of probabilities)
Information from the task analysis can be organised to suit thequantification technique (e.g. describe activities with HEARTgeneric tasks in mind)
Quantification must be based on detailed qualitativeunderstanding
Capability Management Process for HRA
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Prospective Analysis
HRA(concept)
HRA(design)
HRA(validate)
Deliverablesrequired
ProcessAcceptance
CUSTOMER
Liaison
Marketwatch
SAFETY TEAMOwn, Maintain, ManageHRA Processes
Retrospective Analysis PRODUCT
Benchmarking
ExternalSourcesOther
techniques/external
databases/review/researc
h
certify
accident/incident
Design
HRA Data
SafetyAssessmentReporting
Experience/LessonsLearnt
Maturity
Productdesign &
developmentprocess
BU REPS
Bestpractice,peerreview
Knowledge CaptureReporting and debriefing
FeedbackHE assessors, safety team
MeasuresCriteria (e.g. time, labour
involved, etc.)
Training
Process
DatabaseHRA data, lessonslearnt, assessments,techniques
HRSelect &deployteam
Recordingmgt.
Data &process
Processmgt.
Refine,
tailor, what& when
SystemRequirements
Regulatory Authorities
Human Reliability Assessment - Summary
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Human Reliability Assessment Summary
Understand the actions being investigated Use a structured approach to investigate and represent
the actions (task analysis)
Consider the level of detail needed (compare with
description detail of available data)
Understand the failure criteria
Select an appropriate technique(s)
Represent the identified errors in a risk model
Objectives Re-visited
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Objectives Re visited
Understand the Human Reliability Assessment process
Gain practical experience of a simple task analysis
Gain practical experience of error identification
Gain practical experience of risk models Gain practical experience of quantifying error
probabilities in a simple example
Useful References
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Useful References
The following books provide a comprehensive guide toHuman Reliability Assessment and Task Analysis
A Guide to Practical Human Reliability Assessment,
Barry Kirwan (1994),
Taylor and FrancisISBN 07484-0111-3
A Guide to Task Analysis,
Barry Kirwan & Les Ainsworth (1992),
Taylor and Francis
ISBN 07484-0058-3
