Constraints and Errors

An ounce of prevention…

Outline

Part 1 comments Recap Norman About errors Mistakes and slips Error prevention guidelines Error recovery guidelines

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

Recall: Norman Design Concepts

Affordances Visibility Conceptual models Mapping Feedback Constraints

Affordances

Affordances

Mappings: Input

Mappings: Output

Constraints

Limitations on what can be donePhysical - keysSemantic - menu grayingCultural - ColorsLogical - When all above don’t apply

Constraints

Simple Example

What if both sides were “big” and you had toremember which side the “small” one went into?

Electric plugs

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

Errors - Three Considerations

Avoiding and preventing Identifying and understanding Handling and recovering

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)

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

User-Computer Dialog

Three phasesRead-scan phase -- Perceptual errorsThink phase -- Cognitive errorsRespond phase -- Motor errors

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

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

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)

Mistakes and Slips

Mistakes (hopefully avoidable) Wrong intentionIncorrect mental model Novice behavior

Slips (unavoidable)Wrong executionSkilled behavior

Moral …

… slips happen

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

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”

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

What to do?

As much as possiblePrevent mistakesAllow recovery from slips

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

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!

Great Examples

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

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

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?

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

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.

How Does This Example Fit In?

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 ?

Error Message Wording - Tone

Sorry, command not recognized Command not recognized :-( Command not recognized Command not recognized!!

Or Even Worse

Illegal command Illegal command! ILLEGAL COMMAND !@#& ILLEGAL COMMAND!

Which may suggestto some users …

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)

Assignment 7

Think of 3 possible errors in your project applications

List how you might prevent or recover Develop scenario based on one error.

Storyboards!