Human Error Lecture

Human Error

Anna L Cox | University College London | anna.cox@ucl.ac.uk

overview

• a basic taxonomy of human error
• theory: memory for goals
• factors that increase error
• reducing error


Why study human error?

• practically: to avoid catastrophic accidents.
• theoretically: to predict when one errs.
• Cog Sci: it‟s the same system that produces
correct behaviour most of the time! Tells us a
lot about human cognition.


Beatson Oncology Case
• LN, age 15
• chemotherapy & 19 of 20 or 21 planned
radiotherapy treatments
• Because the tumor type, location, and extent
and the patient size, age, and medical
condition vary, the treatment for each patient is
unique
• Each dose 58% HIGHER than it should have
been….and no one noticed
• Scaling factor wasn‟t entered

Blame
• "the error ... was procedural and was not
associated in any way with faults or
deficiencies in the Varis 7 computer system"
(Johnston, 2006; ii).
• “... at no point in the investigation was it
deemed necessary to discuss the incident with
the suppliers if this equipment since there was
no suggestion that these products contributed
to the error.” (Johnston, 2006; 2)


Accidents due to Human Errors
Human Error Others
100%

80%

60%

40%

20%

0%
Petrochemical Medicine Worldwide Jet US nuclear Automobiles Air traffic
plants cargo power plants control


Annual Death Rates in US
100000 98000

One jumbo jet
crash every day

50000 43649

14986
3959
329
0
commerical drowning deaths from motor vehicle deaths from
aviation deaths falls deaths medical
deaths errors


Denise Melanson
• infusion pump
containing a four day
dose of two
chemotherapy drugs,
5-fluorouracil and
cisplatin, to administer
to herself at home.
• pump dispensed the
dose in four hours and
not four days


Blame

• The nurse incorrectly calculated the dose
• A second nurse checked the calculation but
didn‟t notice the error
• The drug bottle first displayed the per 24hr
dose, (and then, in brackets the per 1hr dose)


OptiClik


JB’s video

http://www.youtube.com/watch?v=Hm7k0TRaPHI


A system fails to achieve the
intended outcome in a planned sequence
of mental or physical activities

Human Errors

Slips Mistakes

Incorrect execution of Correct execution of
a correct action sequence an incorrect action sequence


Human Errors

Slips Mistakes

Execution Evaluation Knowledge Rule
Slips Slips Based Based

•--------- •--------- •--------- •---------
•--------- •--------- •--------- •---------
•--------- •--------- •--------- •---------

Norman‟s Action Theory
Delete file

Goal
Use delete key
Intention Evaluation Form sub-goal

Select file + hit
delete” Action Nothing happened
Interpretation
Specification

Use mouse and Screen doesn‟t
Execution Perception change
keyboard to
perform action User Activities

sequence
Physical System

System Activities


Mistakes

Goal
Intention Evaluation

Action
Interpretation
Specification

Execution Perception
User Activities

Physical System

System Activities


Mistakes

Knowledge-Based Rule-Based

• Faulty conceptual knowledge • Misapplication of good rules
• Incomplete knowledge • Encoding deficiencies in
• Biases and faulty heuristics rules
• Incorrect selection of knowledge • Action deficiencies in rules
• Information overload • Dissociation between
knowledge and rules


Slips
Execution Slips Evaluation Slips

Goal
Intention Evaluation

Action
Interpretation
Specification Interruptions
Execution Perception
User Activities

Physical System

System Activities


Capture Slip: automatic activation of a well-
learned routine that overrides the current
intended activity .

“I intended to pick up
my prescription on my
way home. I drove
home directly.”


Double Capture Slip: The unintended
activation of a related strong action schema.

“I meant to take off
only my shoes, but
took my socks off as
well.”


Omission Slip: due to interruptions.


Loss of Activation Slip


Description Slip: incomplete or ambiguous
specification of intention that is similar to a
familiar intention.


Associative Activation Slip: activation of
similar but incorrect schemas.


Perceptual Confusion Slip


Repetition of Action Slip: repetition of an
correctly performed action.

A nurse repeated
radiation therapy to
a patient three
times in a row, due
to poor feedback.
The patient died
three months later.


Cross-talk Slip (concurrent): action
components are exchanged between two or
more concurrent actions.

English writing Dutch reading
Goal Goal

Intention Evaluaton
i Intention Evaluaton
i

Action Action
Interpret tion
a Interpret tion
a
Specifcation
i Specifcation
i

Executi n
o Perce
ption Executi n
o Perce
ption

Physical
Systems Physical
Systems


Cross-talk Slip (sequential): action
components are exchanged between two or
more sequential actions.
“I had just finished talking on the phone when my secretary
ushered in some visitors. I got up from behind the desk and
walked to greet them with my hand outstretched saying „Smith
speaking‟.”
Talking on the phone (previous) Greeting (current)

Goal Goal

Intention Evaluaton
i Intention Evaluaton
i

Action Action
Interpret tion
a Interpret tion
a
Specifcation
i Specifcation
i

Executi n
o Perce
ption Executi n
o Perce
ption

Physical
Systems Physical
Systems


Applying Norman’s Action Cycle to
Error Classification

• Errors are sometimes made when
programming infusion pumps
• These errors might occur when entering
numbers
• What causes these errors and when to they
occur?


Number Entry Error Taxonomy


#errordiary

• We‟re tweeting when we make an error
• Have a go at classifying these errors using
Norman‟s action cycle
• Tweet when you make an error


issues
• Norman relied on self-report data of errors
- gives a sense of common everyday errors
- no reliable estimate of frequency of error
- cannot determine cause of error
• move to the lab
- difficulty is that errors are normally infrequent
- this brings about issue for how to operationalize design
and conduct statistical analysis - self-monitoring
- slip errors occur with routine, procedural tasks
- critical to train participants to criterion


post-completion error
• routine, procedural task
- goal-directed task
- many subtasks
• post-completion step
- the final step in a procedure
- completed after the goal is achieved
• post-completion error (PCE)
- missing the post-completion step
- US ATM, photocopying, filling petrol, etc


example: photocopying


memory for goals

Threshold

Time

• goals are declarative memory representations defined by activation
• most active goal directs behavior
• activation declines over time, rehearsal can strengthen goal activation
• associative links between goals propagate activation through network
• completing ‘make copies’ lessens activation of ‘remove original’

Working memory capacity & workload


experiment
• memory for goals predicts that working
memory load should mediate likelihood of error
• seminal study: Byrne & Bovair (1997)
- working memory capacity: with larger capacity the goal
can be actively rehearsed, making error less likely
- memory load: with increased load the main goal will
decay more rapidly so high chance of error because
post-completion step less likely to reach activation
threshold


Byrne & Bovair’s results


Interruptions


Interruptions in the real world

• Nurses are frequently
interrupted during their duties
• 20% of the time they don‟t go
back to the task they were
working on when they were
interrupted
• Sometimes it‟s because that
task is redundant
• Sometimes it‟s because they‟ve
forgotten
• What are the implications for
patient care?

• Could argue that she wasn‟t paying attention
• She was blamed for not looking at the device

• But what about when we do direct our attention
to the device again
• Does an interruption have any effect on what
happens next?


effect of interruption
• Li et al. (2008):
multi-step
procedural task
making doughnuts
with purposeful
post-completion
step
• interrupted to switch
tasks to pack
doughnuts


error rate

Interruption position


Reducing errors (I)

• cues


reducing errors

• system redesign
- post-completion errors can often be designed out
- e.g., ATM: card then cash vs. cash then card
- but not always feasible for large complex systems
• provide explicit cues
- must be visually salient, just-in-time, and meaningful
- cues that are not specific are ineffective
- habituation is a concern


visual cueing: Chung& Byrne (2008)


visual cueing, revisited


cue effectiveness


But…

• Not necessarily the perfect design solution
• The „just-in-time‟ aspect can make it hard to
design in
• And people become habituated to cues that
are in the world all the time and so they stop
working


Reducing errors (II)

• Slow down!


Error Rate (%)

Interruption Position
data from Li, S.Y.W., Blandford, A., Cairns, P., & Young, R.M.
(2008). Journal of Experimental Psychology: Applied, 14, 314-
328.

distribution of errors

• working on routine task
• interruption occurs
in between subtasks
• on resumption most
common error is to
repeat or skip a step
• this is because of
competition between
goals at time of
retrieval
data from Trafton, J. G., Altmann, E. M., & Ratwani, R. M., (2009). A memory
for goals model of sequential action. International Conference of Cognitive
Modeling, 2009.

Speed-accuracy tradeoff
• some errors are caused by failure of memory
• memory is sensitive to changes in speed-
accuracy tradeoff criterion
• error rate should be reduced if people invest
time in thinking about where they were before
resuming
• we use an enforced lockout procedure


lockout reduces error rate
Error rate (%)

Condition


interruption-only trials
Frequency

Resumption time (seconds)


find out more


Lockouts in design?

• What would YOU do if you were locked out of
a system after an interruption?


implications for design
• errors will occur, even with skilled users
• errors reduced by good system design
- well structured tasks
- few interruptions
- low memory demand
- easy to select buttons
- salient display (particularly mode indicators)
- certain visual cues can be useful
• aid recovery from errors
- make actions reversible
- make the results of each action apparent

Summary
• Slip errors occur
infrequently – but are
persistent
• Increasing speed => • Cues: have to be “just-in-
increasing error rate time” and very aggressive
• Interruptions => increasing • Take your time – slow
error rate down!

• …err what‟s it like on a
hosptial ward?? 


reading
• core
- Byrne, M. D., & Bovair, S. (1997). A working memory model of a common
procedural error. Cognitive Science, 21, 31-61.
- Norman, D.A. (1981). Categorization of action slips. Psychological Review, 88,
1-15.

• supplementary
- Chung, P.H., & Byrne, M.D. (2008). Cue effectiveness in mitigating
postcompletion errors in a routine procedural task. International Journal of
Human-Computer Studies, 66, 217-232.
- Li, S.Y.W., Blandford, A., Cairns, P., & Young, R.M. (2008). The Effect of
interruptions on postcompletion and other procedural errors: An account based
on the activation-based goal memory model. Journal of Experimental
Psychology: Applied, 14, 314-328.
- Ratwani, R.M., McCurry, J.M., & Trafton, J.G. (2008). Predicting
postcompletion errors using eye movements. In Proceedings of CHI’08 (pp.
539-542). New York, NY: ACM Press.
- Reason, J. (1990). Human Error. Cambridge University Press.

Microwave Racing video

http://www.youtube.com/watch?v=Bzy5hVvbei8


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