lecture 12: errors - umd computer science department … · spring 2010 cmsc 434 introduction to...

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Lecture 12: Errors

March 22

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Medical Usability: How to Kill Patients Through Bad Design

•  Under dosage •  Double dosage •  Wrong patient •  Wrong date •  Medical history ignored •  Post-it notes

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Hospital’s order-entry system

•  Misleading default values –  10 mg

•  No feedback, new commands not checked against the old ones –  Double dosage

•  Poor readability –  Wrong patient selected

•  Data description errors –  Tomorrow

•  Memory overload –  Too many screens for a patient’s medical history

•  Overly complicated overflow –  Different from Hospital's actual workflow

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Lecture objectives

•  Identification –  Identify the variety of errors people make

and their possible causes

•  Prevention – Discuss guidelines to prevent the errors

from happening by reducing the causes

•  Intervention – Discuss ways to recover from errors when

they do happen

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Error identification

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Errors people make in taking pictures

•  Wanting to take pictures but taking videos instead

•  Wanting to snap a picture but turning off the camera instead

•  Taking pictures with the lid still covering the lenses

•  Facing the wrong direction •  Sending enormous files by emails •  Covering the lenses by fingers •  Taking pictures with shaky hands •  Not holding the snap button long enough to

trigger a snap

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Error classification

•  When? – Perceptual errors – Cognitive errors – Motor errors

•  How? – Mistakes – Failures – Slips and lapses

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Perceptual errors

•  Causes: – Misleading perceptual cues – Sensory overload

•  Examples: –  I didn’t notice the battery indicate turns red. –  I didn’t realize these buttons are different. –  I thought this underlined sentence is a link. –  I thought I heard the sound of someone

sending me an IM.

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Cognitive errors

•  Causes: – Complex decision process – High load on memory

•  Examples: – The password I remembered was wrong. –  I added the numbers wrong in my head. –  I didn’t know I was in the video mode.

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Cognitive errors

•  Causes: – Complex decision process – High load on memory

•  Examples: – The password I remembered was wrong. –  I added the numbers wrong in my head. –  I didn’t know I was in the video mode.

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Motor errors

•  Causes: – Unnatural motor movement •  Cross hand

– Similar motor sequences – Pressured for speed – Tricky hand-eye coordination – Special motor skills (typing, hand drawing)

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Motor errors

•  Examples: –  I typed the wrong key. –  I pressed the wrong button. –  I double clicked instead of single clicked. –  I checked the wrong box. –  I typed one too many letter. –  I can’t get the mouse to where I want.

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Failures

•  Causes: – Know what to do but difficult to do it

successfully

•  Examples: – Putting a thread into the eye of a needle – Looking for a needle in the haystack – Adding numbers in the head – Pulling off a quadruple click

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Mistakes

•  Causes: – Do the wrong thing for the goal – Apply a rule in a wrong situation – Make a bad decision

•  Users may not be aware of the mistake •  It can be a learnability or memory issue.

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Mistakes

•  Examples: – Sending a gigantic photo via email – Ordering a product from a shady vendor – Deleting files by removing shortcuts from

the desktop – Forgetting to turn cell phones to vibration

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Slips and lapses

•  Causes: – Working memory failure – Do something familiar but miss a few steps

•  Slip –  failure of execution

•  Lapse –  failure of memory

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•  Unintentional •  Users realize right away •  More common than mistakes •  Slow users down

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Six types of slips and lapses

1.  Capture errors 2.  Description errors 3.  Data driven errors 4.  Associate activation 5.  Loss of activation 6.  Mode errors

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1. Capture errors

•  A familiar sequence of actions captures an unfamiliar sequence of actions.

•  Examples: – Get off the exit to work prematurely on a

weekend grocery trip – Open up a web browser and find yourself

going to Facebook instead of where you meant to go

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2. Data-driven error

•  An automatic action triggered by arrival of sensory info which intrudes into the normal action

•  Parameters in STM wrongly replaced by new sensory info

•  Examples: – Wanting to say X but instead saying Y

because you heard someone nearby yelling Y

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3. Description errors

•  Actions have similar descriptions •  When performing one action, one

mistakenly perform a different action that fits the description.

•  Description can be visual, spatial, or semantic.

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3. Description errors

•  Wanting to maximize a window but instead minimizing it – Description: ?

•  Wanting to save a file but instead opening up the save-as dialog box – Description: ?

•  Wanting to reply to sender but instead reply to all – Description: ?

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3. Description errors

•  Wanting to maximize a window but instead minimizing it – Description: click on the button at the upper-left

corner (spatial similarity) •  Wanting to save a file but instead opening

up the save-as dialog box – Description: click on the button that looks like a

floppy disk (visual similarity) •  Wanting to reply to sender but instead reply

to all – Description: click on the button for reply

(semantic similarity)

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4. Associative activation

•  Internal thoughts and associations trigger the wrong action

•  Wrong reflexes •  Example: – Phone rings yell “come in”. – Dismiss an important message box

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5. Loss of activation

•  Forgot the goal in the middle of a sequence of actions performed to fulfill the goal.

•  Examples: – Forgot what we went into the room for. – Forgot why we visited a website – After starting Skype, forgot whom we wanted

to call – Go to Google but forgot want we wanted to

search

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6. Mode error

•  Do action in one mode thinking you’re in another.

•  Examples: – Typing password not knowing the Cap Lock

is on – Deleting all files but in the wrong directory. – Typing gibberish in the wrong language

mode.

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Exploiting user errors (Bad)

•  Phishing •  Fake anti-virus program •  Internet button next to call button

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Activity: What kinds of slips and lapses?

1.  Wanting to take a picture but shooting a video instead

2.  Typing the password in the field for the login name

3.  After installing a software, forgetting why you did it

4.  Clicking on “send” without first attaching a filf 5.  Typing ‘ls’ in a Windows terminal 6.  Closing a popup immediately upon seeing it

and prematurely terminating a process 7.  After seeing a new email notification, typing the

sender’s name in a word document

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Error prevention

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Tragedy that can be avoided

•  People running down the stairs but found themselves trapped in the basement …

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Forcing function

•  Interlock •  Lockin •  Lockout

•  Behavior shaping constraints •  Physical vs. Software

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Interlock

•  Force operations to take place in a proper sequence

•  Real-world examples: – Cars that force us to shift to P before we can

turn the engine off

•  UI examples: – ATMs that force us to remove ATM cards

before before we can proceed

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Lockin

•  Keep an operation active, preventing someone from accidentally stopping it prematurely

•  Real-world examples •  UI examples: – soft on/off switch

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Lockout

•  Require efforts to unlock a feature to prevent accidental access

•  Real-world examples: the stopping bar in the stair to the basement, the safety-pin of a grenade

•  UI examples: – Mac’s lock button

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Errors where forcing functions may apply

•  I forgot to remove the cartridge before turning my Nintendo off. –  Interlock

•  I forgot to remove the lid off the lenses before taking pictures –  Interlock

•  I accidentally close the window before the download is completed –  Lockin

•  I accidentally formatted my hard drive. –  Lockout

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Errors summary

•  When? (read-scan, think, act) –  Perceptual errors –  Cognitive error –  Motor errors

•  How? (intentional vs. accidental) –  Failures –  Mistakes –  Slips and lapses

1.  Capture errors 2.  Description errors 3.  Data driven errors 4.  Associate activation 5.  Loss of activation 6.  Mode errors

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Preventing general errors

Avoid the causes listed earlier …

•  Perceptual – Avoid misleading visual cues

•  Cognitive – Avoid memory load

•  Motor – Avoid unnatural motor movement

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Preventing mistakes

•  Hard to do through design – We can’t prevent users from making bad

decisions by intention

•  Possible solutions: – Provide “reasonableness” check – Provide help and documentation

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Preventing lapses

1.  Capture errors 2.  Description errors 3.  Data driven errors 4.  Associative activation 5.  Loss of activation 6.  Mode errors

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1. Preventing capture errors

•  Avoid habitual action sequences with identical prefixes

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2. Preventing description errors

•  Avoid actions with very similar descriptions

•  Keep dangerous commands away from common ones – E.g., save and delete should not be together

•  Provide contrast

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3. Preventing data-driven errors

•  Avoid distracting sensory information •  Reduce the need to keep data in the

working memory (so it won’t get replaced)

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4. Preventing associative activation errors

•  Avoid similar stimulus or notification for different actions

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5. Preventing loss of activation

•  Keep procedures short – so users won’t lose track of the steps

•  Minimize interruptions – so users won’t get distracted and forget

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6. Preventing mode errors

•  Eliminate modes or provide visible cues •  Minimize shared actions across modes •  Use spring loaded or temporary modes. – Examples: •  drag-and-drop by holding down mouse button •  batch-selection by holding shift

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Confirmation dialog

•  Limitations: – People tend to explain away errors. – Reduces the efficiency – Habitual dismissal reduces its effectiveness

when something serious really happens

•  Examples: – Engine check light – False alarms – Web site security warning

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Activity: What kinds of slips and lapses?

1.  Wanting to take a picture but shooting a video instead

2.  Typing the password in the field for the login name

3.  After installing a software, forgetting why you did it

4.  Clicking on “send” without first attaching a filf 5.  Typing ‘ls’ in a Windows terminal 6.  Closing a popup immediately upon seeing it

and prematurely terminating a process 7.  After seeing a new email notification, typing the

sender’s name in a word document

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Error recovery

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Guidelines

•  Provide undo •  Provide cancel •  For form input errors, help users locate

the fields that need to be fixed •  Provide context-sensitive help •  Comfort the users

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Error messages

•  Best error message is none at all •  Be precise •  Restate user’s input •  Speak the user’s language •  Give constructive help •  Be polite, avoid loaded words •  Avoid frivolous error messages

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•  The mouse cursor is outside of the visible screen area.

•  A string is typed in the font size selection text field!!

•  Error code = 24 •  Your action has caused the program to

crash.

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•  Fatal error saving file •  The file can not be saved. •  There is an error saving “xxx.doc”. •  There is an error saving “xxx.doc”

because of the format. •  There is an error saving “xxx.doc”

because of the format. Please make sure it is an image file.

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