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Constraints and Errors
An ounce of prevention…
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Outline
Part 1 comments Recap Norman About errors Mistakes and slips Error prevention guidelines Error recovery guidelines
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Project part 1 comments
Think of the overall goals, motivations, and desires of the usersPhotos can be highly emotionalLook for inspiration in physical worldMay suggest different aspects of the
design space, metaphors, etc. Narrow down usability criteria
Eventually want to come up with testable statements
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Recall: Norman Design Concepts
Affordances Visibility Conceptual models Mapping Feedback Constraints
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Affordances
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Affordances
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Mappings: Input
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Mappings: Output
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Constraints
Limitations on what can be donePhysical - keysSemantic - menu grayingCultural - ColorsLogical - When all above don’t apply
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Constraints
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Simple Example
What if both sides were “big” and you had toremember which side the “small” one went into?
Electric plugs
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Knowledge in Head vs. Knowledge in the World
Head Not easy to retrieve Learning required, good
conceptual model makes easier
Can be very efficient Not easy first time Aesthetics do not need
to make info visible
World Easy to retrieve No learning, only
interpretation Use slowed by need to
find the info to interpret Easy for first time Can be cluttered or
inelegant
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Errors - Three Considerations
Avoiding and preventing Identifying and understanding Handling and recovering
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Why errors are important
Errors are unavoidableTo err is humanMaking mistakes is part of learning
Designer’s responsibilityUnderstand why errors occurMinimize likelihoodAllow for recognition of error and graceful
recovery (forward or backward)
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Example Studies – Errors Happen! 170 experienced UNIX users over 9 days
Individual commands had error rates of 3-50%
300 security system users over 20 months 12,117 error messages Most common 11 errors -> 65% 2517 involved repeated errors (with no non-
errors in between) within 10 minutes Bad error recovery/help
Kraut et al, CHI ‘83
Mosteller & Ballas, Human Factors ‘89
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User-Computer Dialog
Three phasesRead-scan phase -- Perceptual errorsThink phase -- Cognitive errorsRespond phase -- Motor errors
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Perceptual Errors
Result from insufficient or poor perceptual cuesDisplay of objects that are visually
similarInvisible or poorly expressed statesFailure to capture user’s attentionLack of perceivable feedback
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Cognitive Errors
Caused by taxing the memory and problem solving capabilitiesTax recall memoryLack of or poor mnemonic aidsInconsistencyLack of context or status info
• e.g., where came from in a menuMental calculations and translations
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Motor Errors
Taxing the eye-hand coordination and motor skillsAwkward motor movementsHighly similar motor sequences
• e.g., double click, click
Pressure for speedRequire a high degree of hand-eye
coordinationRequiring special types of motor skills
(type)
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Mistakes and Slips
Mistakes (hopefully avoidable) Wrong intentionIncorrect mental model Novice behavior
Slips (unavoidable)Wrong executionSkilled behavior
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Moral …
… slips happen
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Types of Slips
1. Capture error - Continue frequently done activity instead of intended one (similar starts)Confirm deletion of file instead of cancel
2. Description error - Intended action has much in common with others possible (usually when distracted, close proximity)shift key & caps lock key
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Types of Slips
3. Data driven error - Triggered by arrival of sensory info which intrudes into normal actionCall to give someone a number, dial that
number instead 4. Associative activation - Internal
thoughts and associations trigger actionPhone rings, yell “come in”
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Types of Slips
5. Loss of activation - Forgetting goal in middle of sequence of actionsStart going into room, then forget why
you’re going there 6. Mode errors - Do action in one mode
thinking you’re in anotherDelete file, but you’re in wrong directory
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What to do?
As much as possiblePrevent mistakesAllow recovery from slips
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Error Prevention Guidelines
Eliminate modes or provide visible cues for modes - no invisible modes
Use good coding techniques (color, style)
Maximize recognition, minimize recall Design non-similar motor sequences
for commands Minimize need for typing
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Error Prevention Guidelines
Test and monitor for errors and engineer them out
Allow reconsideration of action by user (e.g., removing file from trash)
Avoid ambiguous and unclear prompts and messages - as in next set of examples!
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Great Examples
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Error Recovery Guidelines
Provide appropriate type of responseGag - Prevent user from continuing
• Erroneous loginWarn - Warn user an unusual situation
is occurring• Bell or alert box
Nothing - Don’t do anything (Careful, user must determine problem)
• move file to bad place
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Error Recovery Guidelines
Responses (continued)Self-correct - Guess correct action &
do it• Spell-check correction
Dialog - System opens dialog with user
• Go into debugger on run-time crash
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Error Recovery Guidelines
Provide undo function Provide cancel function from
operations in progress Require confirmation for drastic,
destructive commands Provide reasonableness checks on
input dataDid you really mean to order 5000?
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Error Recovery Guidelines
Return cursor to error field, allow fix Provide some intelligence
Guess what they wanted to do Provide quick access to context-
sensitive help
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Error Message - What to Say Error: Error code -37 Description: Disk full Prescription: Disk full; recover disk
space Prescription + aid: Disk full; recover
space by deleting files or defragmenting
Prescription + offer: Disk full; proceed with disk defragmentation? Otherwise delete files.
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How Does This Example Fit In?
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Error Message Wording - Vocabulary Problem with previous example -
some users will not know what defragmentation means!!
VocabularyUser-orientedDefined in advance for commonality
throughout all messages (in style guide)
Alternatives to defragmentation ?
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Error Message Wording - Tone
Sorry, command not recognized Command not recognized :-( Command not recognized Command not recognized!!
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Or Even Worse
Illegal command Illegal command! ILLEGAL COMMAND !@#& ILLEGAL COMMAND!
Which may suggestto some users …
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Implications for design
Scenarios should include errors !!! Distinguish between skilled and
learner errors Uncover errors in the existing system
how do people self-monitor (cheat sheets)
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Assignment 7
Think of 3 possible errors in your project applications
List how you might prevent or recover Develop scenario based on one error.
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Storyboards!