Transcript Human reliability in engineering systems

Human reliability in engineering systems
Presented by:
Zaniar golabi & sina masihabadi
Professor:Dr.jobin ghayoor
introuduction
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In 1958 williams recognized that human element reliability must
be included in overall system reliability.
In 1960 proved that human error is the cause for %20 to %50
of all equipment failure.
In 1962,a database known as DATA STORE containing time and
human performance reliability estimates for human engineering
design features was established.
In 1973,ieee transaction on reliability published a special issue
on human reliability.
In 1980,a selective bibliography on human reliability was
published covering the period from 1958 to 1978.
The first book on human reliability entitled human reliability
with human factors appeared in 1986.
Terms and definitions
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Human error:this is the failure to perform a given task that could
resault in the disruption of scheduled operations or damage to
property and equipment.
Human performance: this is a measure of human functions
subject to specified conditions.
Continuous task: this is a task that involves some sort of tracking
activity: for example, monitoring a changing situation.
Human reliability: this is the probability of accomplishing a task
successfully by the human at any required stage in system
operations within a stated minimum time.
Human error occurrence examples
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Human operator accounted for over %90 of the documented air
traffic control system errors.
Over %50 of all technical medical equipment problems are due
to operator errors.
Up to %90 of accidents both generally and in medical devices
are caused by human mistakes.
A study of 23000 defects in the production of nuclear
components revealed that approximately %82 of the defects
were due to human errors.
During the period from june 1,1973 to june 30,1975,401 human
errors occurred in U.S commercial light water nuclear reactors.
Human error occurrence types:
Design errors: these types of errors are the result of inadequate
design. for example the placement of controls and displays so far
apart that an operator is unable to use them in an effective manner.
Operator errors :these errors are the result of operator mistakes and
the conditions that lead to operator errors include lack of proper
procedures ,complex tax ,poor training and operator carelessness.
Assembly errors: these errors occur during product assembly due to
humans.
Inspection errors: these errors occur because of less than %100
accuracy of inspectors.
Maintenance errors: the errors occur in the field due to oversights
by the maintenance personnel.
Installations errors: these errors occur due to various reasons
including using the wrong installation related blueprints or
instructions.
Handling errors: these errors basically occur because of inadequate
storage or transportation facilities.
Causes for the occurrence of human
errors:
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Poor motivation of involved personnel.
Poor training or skill of concerned personnel.
Poor equipment design.
Inadequate or poorly written equipment operating and
maintenance procedures.
Poor job environment: poor lighting ,high/low temperature ,high
noise level ,crowded work space, ,etc.
Inadequate work tools.
Complex tasks.
Poor work layout
Human error occurrence classification:
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decision error: occur when the wrong decision is made after
considering the situation.
Action error: are the result of no action ,incorrect action ,or the
performance of correct action on the wrong object when required.
Transmission error: occur when information that must be passed or
to others is not send ,sent incorrectly ,or sent to the wrong destination.
Checking error: occur when system require checks,the incorrect
checks are made ,checks are omitted ,or correct checks are made on
the wrong object.
Diagnostic errors :are the result of misinterpreting the actual
situation when an abnormal event occurs.
Retrieval errors :occur when required information either from an
individual,an individual memory,or from any other reference source is
not received or the incorrect information is recieved.
Factors that influence on human
performance:
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Reaction to stress
Time at work
Fatigue
Group interaction and identification
Social pressure
Repetitive work
Supervisor’s expectations
Morale
Social interaction
Crew efficiency
Idle time
Stress factors and operator stress characteristics
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Dissatisfied with the current job.
Faced with serious financial problems.
Working with individuals having unpredictable temperaments.
Low chance of promotion from current position.
facing a possible work layoff.
Current job below ability and experience.
Conducting tasks under extremely tight time schedules.
Having health problems.
Excessive demands of superior.
Inadequate expertise to conduct the tasks required in the
current job.
Often tasks work home to meet deadlines.
Having difficulties with spouse/children.
The operator stress characteristics
include:
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short decision making time
Several displays difficult to discriminate.
Requirement to perform steps at high speed
Poor feedback for the determination of accuracy of actions
taken.
Requirement for prolonged monitoring.
Very long sequence of steps required to perform a task.
Requirement to make decisions on the basis of data obtain from
various different sources.
Requirement to operate at high speed more than one control
simultaneously.
Human performance reliability in continuous
time and mean time to human error (mtthe)
measure
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As humans perform time continuous tasks such as
aircraft maneuvering ,scope monitoring ,and missile
countdown, a general expression for human
performance reliability can be developed the same
way as for the development of the general classical
reliability functions.
Human reliability evaluation
methods
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Probability tree method: this is used to perform task
analysis by diagrammatically representing critical
human actions and other events associated with the
system.
Fault tree method: this special case has been
discussed in the class.
Markov method: this is a powerful reliability
engineering tool that can also be used to perform
time continuous human reliability analysis.
Human reliability markov
modeling
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Reliability analysis of a system with human error: this
mathematical model represent a system which can
fail either due to hardware failure or to a human
error.
Reliability analysis of a human performing a time
continuous task under fluctuating environment: this
mathematical method represents a human
performing a time continuous task under fluctuating
environment.
Human error data
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Human reliability predictions are only as effective as the body of
quantitative on which they are made.
The collection and maintenance of human error data are at least
as important as making various types of human reliability
predictions.
Means to collecting human error
data
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Experimental studies.
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Expert judgments.
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Self made error reports.
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Human data recorder.
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Automatic data recorder.
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Published literature.
Human error data
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Specific human error data banks and
sources
Human error data for selective tasks
Brief description of selected human eror
data banks
Specific human error data banks
and sources
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Data store
Operational performance recording and evaluation data system
Nuclear plant reliability data system
Aerojet general method
Aviation safety reporting system
Bunker ramo tables
Safety related operator action program
Technique for establishing personnel performance standards
Brief description of selected
human error data banks
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Data store: the data store was established in 1962 by
american institute for research, pittsburgh and it contains
estimates for time and human performance reliability.
Aviation safety reporting system: originally this databank
was developed by the national aeronautics and space
administration (NASA) and contains information on civil aircraft
accidents.
Operational performance recording and evaluation data
system: this system was developed to collect data on
operational human performance by the U.S navy electronics
laboratory, san diego.
Learning from other fields: PSTN
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FCC-collected data on outages in the US public-switched telephone
network
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metric: breakdown of customer calls blocked by system outages
(excluding natural disasters). Jan-June 2001
Human error accounts for 56% of all blocked calls
9%
22%
Human-co.
Human-ext.
5%
Hardware Failure
Software Failure
47%
17%
Overload
Vandalism
Learning from other fields:
PSTN
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PSTN trends: 1992-1994 vs. 2001
Cause
Trend
Minutes (millions of customer minutes/month)
1992-94
2001
Human error:
company
98
176
Human error:
external
100
75
49
49
15
12
314
60
5
3
Hardware
Software
Overload
Vandalism
MARKOV METHOD
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Assumptions:
The probability of a transition occurrence from one state to
another in finite time Δt is λΔt .The parameter λ in our case is
the constant human error rate.
The probability of two or more transitional occurrence in Δt is
negligible.
All occurrences are independent of each other.
MARKOV METHOD
System has two states.The she state 0 represent human
performing the time continuous task normally and state 1
denotes human committed error.
By using the MARKOV METHOD we can obtain the probabilites of
the system being in state 1 or 0 at time t for known human error
rate.
P0(t+ Δt)=P0 (t)(1-λ Δt)
P1(t+ Δt)=P1 (t)(λ Δt)+P1(t)
MARKOV METHOD
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Rearranging and taking limits:
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dp0(t)/d(t)=- λ p0(t)
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dp1(t)/d(t)= λ p0(t)
At time t=0:
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p0(0)=1
p0(t)=exp(-λ t)
p1(t)=1- exp(-λ t)
and
p1(0)=0
so:
Thus the human reliability R(t) is given by:
R(t)=p0(t)=exp(-λ t)
MARKOV METHOD
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By integration over the time interval [0,∞]:
MTTHE=∫R(t) dt= 1/λ
HUMAN RELIABILITY MARKOV
MODELING:
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This mathematical model represents a system which can be failed due
to a hardware failure or a human error.
Both failure rates are constant.
We have the following symbols:
λh:the constant human error rate
λnh:the constant nonhuman error rate
Pi(t): the probability that system is in state i at time t
i=0
system operating normally
i=1
system failed due to nonhuman error
i=2
system failed due to human error
HUMAN RELIABILITY MARKOV
MODELING:
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By applying the Markov method:
dp0(t)/d(t)+ (λh+ λnh)p0(t)=0
dp1(t)/d(t)- λnh p0(t)=0
dp2(t)/d(t)- λh p0(t)=0
Solving the above equations we have:
P0(t)=exp(-(λh+λnh)t)
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P1(t)= λnh/(λnh + λh)[1-exp(-(λh+ λnh)t)]
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P2(t)= λh/(λnh + λh)[1-exp(-(λh+ λnh)t)]
The system reliability R(t) with human error is:
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R(t)=p0(t)=exp(-(λh+λnh)t)
HUMAN RELIABILITY MARKOV
MODELING
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By integrating over the time interval [0,∞]:
MTTF= ∫R(t)dt=1/λh
Refrences:
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Dhillon,B.S,design reliability,fundamentals and
applications,CRC press,1999
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J. Reason, Human Error, Cambridge, 1990
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www.eliminatemistakes.com
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Human error in medicine,marc green ,2003
Thanks for your attention