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CCT384 – Universal Design and AccessUD Principle: Tolerance for Error
Week 3
From previous week
Designing Labs for Peoples with Disabilities
http://www.washington.edu/doit/Video/Wmv/temp/equal.asx
The design minimizes hazards and negative consequences of accidental actions.
Principle 5: Tolerance for Error
Principle 5: Tolerance for Error
The design minimizes hazards and negative
consequences of accidental actions.
“CAUTION: It is not recommended that children or pets regularly drink water from the toilet, even though the bowl water is not harmful to children or pets.”
Label on toilet bowl cleaner bottle
Design of Everyday Things
About Donald A. Norman
BS and EECS from MIT
Ph.D. in psychology from UPenn
Centers for Cognitive Studies at Harvard
professor emeritus of cognitive science at UCSD
VP of Advanced Technology Group at Apple; HP
co-founder of Nielsen Norman Group (usability consulting company)
professor of computer science at Northwestern
About the Book
first published in 1988original title: “The Psychology of Everyday Things”
User-Centered Designstructure of tasksmaking things visiblegetting the correct mappingexploiting the powers of constraintdesigning for errorexplaining affordances and seven stages of action.
Slips versus Mistakes
Recall: Human errors can be classified into slips and mistakes
Can understand using Norman’s gulf of execution SLIP: If you understand a system well you may know exactly what
to do to satisfy your goals: you’ve formulated the correct action. But you may fail to execute that action correctly (mis-type, press the wrong button)
MISTAKE: If you don’t know the system well you may not even formulate the right goal. (Example: you may pick the magnifying glass icon thinking it is the ‘find’ function, when it actually zooms the text).
Both may be corrected for, and designed around.
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Errors in User-Computer Dialog
Three phases Read-scan phase -- Perceptual errors Think phase -- Cognitive errors Respond phase -- Motor errors
Can generally lead to either slips or mistakes
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Perceptual Errors
Result from poor perceptual cues Display of objects that are visually similar Invisible or poorly expressed states Failure to capture user’s attention Lack of perceivable feedback
Perceptual Errors12
Are perceptual errors likely here?
Tallly Ho Uniforms
Cognitive Errors
Caused by taxing memory and thinking Tax recall memory Poor mnemonic aids Inconsistency Lack of context or status info
e.g., where came from in a menu Mental calculations and translations
Cognitive Errors
Are cognitive errors likely here?
Motor Errors
Taxing the motor skills Awkward movements Highly similar motor sequences
e.g., double click, click Pressure for speed Require a high degree of hand-eye coordination Requiring special types of motor skills (type)
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Motor Errors
Lots of errors are likely here!!
Example Studies
170 experienced UNIX users over 9 days Individual commands 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
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Kraut et al, CHI ‘83
Mosteller & Ballas, Human Factors ‘89
Slips
Automatic (subconscious) error that occurs without deliberation
Examples?
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Types of Slips
1. Capture error Continue frequently done activity instead of intended one
Type “animation” instead of animate Confirm deletion of file instead of cancel
2. Description error Intended action has much in common with others possible
(usually when distracted, close proximity) ctrl key & caps lock key / Sun & Mac
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Types of Slips
3. Data driven error Triggered by arrival of sensory info which intrudes into normal
action Call to give someone a number, dial that number instead
4. Associative activation Internal thoughts and associations trigger action
Phone rings, yell “come in”
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Types of Slips
5. Loss of activation Forgetting goal in middle of sequence of actions
Start going into room, then forget why you’re going there
6. Mode errors Do action in one mode thinking you’re in another
Delete file, but you’re in wrong directory
Error-handling Strategies
1. Avoid and prevent
2. Identify and understand
3. Handle and recover
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Preventing Errors
Rules of thumb: Preventing slips can be done by analysing users’
interaction with the application, then tweaking screen design, button spacing, etc.
Preventing many mistakes requires that users understand the system better; may require more radical redesign, or perhaps a totally different metaphor
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Error Prevention Guidelines
Eliminate modes or provide visible cues for modes
Use good coding techniques (color, style)
Maximize recognition, minimize recall
Design non-similar motor sequences or 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)
Error Prevention Guidelines
Provide appropriate type of feedback Gag - Prevent user from continuing
Erroneous login
Error prevention
Warn user an unusual situation is occurring Bell or alert box
Error prevention
Nothing - Just don’t do anything (Careful, user must determine problem) Mac: move file to bad place
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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 data Did you really mean to order 5000?
Fall 2006PSYCH / CS 6750
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Error Recovery Guidelines However, before a user can recover, must be able to detect that an error
has occurred Detection: provided by easy visibility, feedback
Other options? Self-correct - Guess correct action & do it
Spell-check correction Dialog - System opens dialog with user
Go into debugger on run-time crash
Query - Ask user what should’ve been done, then allow error action as legal one (“did you mean…?”)
Command language naming error
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Error Recovery Guidelines
Return cursor to error field, allow fix Tell them what to fix, how to fix it
Provide some intelligence Guess what they wanted to do
Provide quick access to context-sensitive help
Fall 2006PSYCH / CS 6750
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Error Handling Example (Web)
Form fill in is the most common error case
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????
User Support (aka “Help)
Line between error recovery and help can be fuzzy
Overarching design principle: must be as unobtrusive as possible
Command Assistance
E.g., on-screen manuals, help commands (“man” on Unix), etc.
Simple and efficient if the user knows what he/she is looking for and is seeking either a reminder or more detailed information
But… What if people don’t know what they’re looking for? What about commands that the user does not know about but
needs? What about commands the user thinks exist but do not?
Command assistance is little help here.
Context-sensitive Help
Move away from placing onus on user to remember the command
Often not very sophisticated
Common examples: Microsoft’s “What’s This?” option Tooltips Clippy (arrrgh…)
What’s the “context”? Just the control itself? (Simple) User’s past history and application state? (More sophisticated)
Wizards and Assistants
Attempt to prevent errors by providing “common paths” through software Safety, efficiency, and accuracy (as long as it’s a supported task)
May be unnecessarily constraining
Guidelines: allow backward movement, show progress indicator
Assistant: Clippy is actually an example of this. A context-sensitive trigger to launch a wizard style interaction
Q: What went wrong with Clippy?
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Mistake-proofing: a preliminary definition
Mistake-proofing is the use of process design features* to facilitate correct actions, prevent simple errors, or mitigate the negative impact of errors.
Mistake-proofing tends to be inexpensive, very effective, and based on simplicity and ingenuity.
It will not make processes free of all errors, nor is it a stand-alone technique that will eliminate the need for any other responses to error.
*these process design features will be referred to as “devices” or “counter-measures”
Design
…Then the methods of reducing risks and hazards are limited to: What can be put on paper and subsequently… What can be embedded in the human brain.
“Knowledge in the head”*
*Source: Donald Norman, The Design of Everyday Things
Design
“human errors can be made irrelevant to outcome, continually found, and skillfully mitigated.”
Can human errors become irrelevant by only changing knowledge in the head?
“Knowledge in the World”*
*Source: Donald Norman, The Design of Everyday Things
To err is human
Have you ever gone somewhere and not remembered why you went there?
Have you ever gone home when you meant to stop at a store?
Why does that happen? How would you prevent it if your life depended on it?
“Be more careful” not effective
“The old way of dealing with human error was to scold people, retrain them, and tell them to be more careful … My view is that you can’t do much to change human nature, and people are going to make mistakes. If you can’t tolerate them ... you should remove the opportunities for error.”
“Training and motivation work best when the physical part of the system is well-designed. If you train people to use poorly designed systems, they’ll be OK for awhile. Eventually, they’ll go back to what they’re used to or what’s easy, instead of what’s safe.”
“You’re not going to become world class through just training, you have to improve the system so that the easy way to do a job is also the safe, right way. The potential for human error can be dramatically reduced.”
Chappell, L. 1996. The Pokayoke Solution. Automotive News Insights, (August 5): 24i. LaBar, G. 1996. Can Ergonomics Cure ‘Human Error’? Occupational Hazards 58(4): 48-51.
A new attitude toward preventing errors:
“Think of an object’s user as attempting to do a task, getting there by imperfect approximations. Don’t think of the user as making errors; think of the actions as approximations of what is desired.”*
*Source: Norman, The design of everyday things. Doubleday 1988.
These approximations are part of Norman’s concept of “knowledge in the head”
A New Attitude toward Preventing Errors
Make wrong actions more difficult
Make it possible to reverse actions — to “undo” them—or make it harder to do what cannot be reversed.
Make it easier to discover the errors that occur.
Make incorrect actions correct.
These outcomes do not occur without design changes
Precise outcomes without precise knowledge or action?
Provide clues about what to do: natural mappings affordances visibility feedback constraints
Natural Mappings: Which dial turns on the burner?
Stove A
Stove B
Affordances: How would you operate these doors?
BA C
Push or pull? left side or right? How did you know?
Affordances: How would you lift this pan?
“SUPPORT THE BOTTOM”
Visibility and Feedback
Visibility means making relevant parts visible, and effectively displaying system status
Feedback means providing an immediate and obvious effect for each action taken.
Constraints: How would you assemble these parts?
Interdisciplinary Approach to Design
Psychology:
Norman recommends designing forcing functions into process: “actions are constrained so that failure at one stage prevents the next step from happening.” “[they] rely upon properties of the physical world for their operation; no special training is necessary”.
“Knowledge in the Head”vs. “Knowledge in the World”
Designing Benign Failures
Quality Management:
Shingo recommends that “when abnormalities occur, shut down the machines or lock clamps to halt operations thereby preventing the occurrence of serial defects.”
With “the idea of discovering errors in conditions that give rise to defects and performing feedback and action at the error stage so as to keep those errors from turning into defects.”
What is Poke-yoke?
A method that uses sensor or other devices for catching errors that may pass by operators or assemblers. 1. Control Approach - Shuts down the process when an error occurs. - Keeps the “suspect” part in place when an operation is incomplete.2. Warning Approach - Signals the operator to stop the process and correct the problem.
Introduction
Shigeo Shingo invented the Japanese concept called Poka-Yoke (pronounced POH-kah YOH-kay)
Poka-Yoke means to mistake proof the process The essential idea of poka-yoke is to design your
process so that mistakes are impossible or at least easily detected and corrected
First Poka-Yoke Device
Shingo suggested a solution that became the first Poka-Yoke device In the old method, a worker began by taking two
springs out of a large parts box and then assembled a switch
Problem: Sometimes the worker failed to put both springs in the switch
In the new approach, a small plate is placed in front of the parts box and the worker's first task is to take two springs out of the box and place them on the plate Then the worker assembles the switch. If any spring
remains on the plate, then the worker knows that he or she has forgotten to insert it
The new procedure completely eliminated the problem of the missing springs
Empty plate
First Poka-Yoke Device
Categories of Poka-Yoke
Poka-yoke devices fall into two major categories: Prevention Detection
A prevention device engineers the process so that it is impossible to make a mistake at all
A classic example of a prevention device is the design of a 3.5 inch computer diskette
Impossible to put in upside down
Prevention Example
Categories of Poka-Yoke
A detection device signals the user when a mistake has been made, so that the user can quickly correct the problem
My car beeps if I leave the key in the ignition
Detection Example
Locking devices on filing cabinet prevents opening multiple drawers which would lead to tipping
Other Poka-Yoke Examples
Gas pumps are equipped with hose couplings that break-away and quickly shut-off the flow of gasoline
Other Poka-Yoke Examples
This rental truck has a door latch which will not allow the loading ramp to slide out while the latch is in the closed
position
Other Poka-Yoke Examples
The Saf-T-Smart valve kit detects when the washing machine is operating and opens the valves allowing water to flow freely. When the washing machine is off the valves are closed, reducing pressure on the hoses and avoiding serious water damage (and returning to a flooded house).
Other Poka-Yoke Examples
The U.S. Consumer Product Safety Commission has recorded 105 deaths that were caused by wheelchairs rolling away from the person.The response? A mistake-proofing device that locks the wheelchair when no one is sitting in it. An unlocking lever on the handle allows the wheelchair to be moved when empty.
Other Poka-Yoke Examples
When pipes are likely to freeze, the common practice is to open faucets slightly and let them drip.
This device automatically opens a valve that discharges just enough water to avoid pipes freezing.
Other Poka-Yoke Examples
Other Poka-Yoke Examples
Limit switches
Interlocks
Jigs
Alignment marks
Go/no-go devices
Electronic eyes
Checklists
Summary
Poka-Yoke means to mistake proof the process Remove the opportunity for error
Improve the process
Make wrong actions more difficult
If you can’t remove the opportunity for error Make it easier to discover the errors that do occur
This is not about punishing workers
Mistake proofing recognizes that every human will make mistakes and tries to set up systems that minimize or eliminate those mistakes.
Causes of Human Errors in Computer Systems
1. Personal factors (35%): Lack of skill, lack of interest or motivation, fatigue, poor memory, age or disability
2. System design (20%): Insufficient time for reaction, tedium, lack of incentive for accuracy, inconsistent requirements or formats
3. Written instructions (10%): Hard to understand, incomplete or inaccurate, not up to date, poorly organized
4. Training (10%): Insufficient, not customized to needs, not up to date
5. Human-computer interface (10%): Poor display quality, fonts used, need to remember long codes, ergonomic factors
6. Accuracy requirements (10%): Too much expected of operator
7. Environment (5%): Lighting, temperature, humidity, noise
Because “the interface is the system” (according to a popular saying), items 2, 5, and 6 (40%) could be categorized under user interface
Causes of Human Errors in Computer Systems
Properties of a Good User Interface1. Simplicity: Easy to use, clean and unencumbered look
2. Design for error: Makes errors easy to prevent, detect, and reverse; asks for confirmation of critical actions
3. Visibility of system state: Lets user know what is happening inside the system from looking at the interface
4. Use of familiar language: Uses terms that are known to the user (there may be different classes of users, each with its own vocabulary)
5. Minimal reliance on human memory: Shows critical info on screen; uses selection from a set of options whenever possible
6. Frequent feedback: Messages indicate consequences of actions
7. Good error messages: Descriptive, rather than cryptic
8. Consistency: Similar/different actions produce similar/different results and are encoded with similar/different colors and shapes
Everyday Examples
New lawn mowers are required to have a safety bar on the handle that must be pulled back in order to start the engine. If you let goof the safety bar, the mower blade stops in 3 seconds or less.
Fueling area of car has three error-proofing devices: 1. insert keeps leaded-fuel nozzle from being inserted 2. tether does not allow loss of gas cap 3. gas cap has ratchet to signal proper tightness and prevent overtightening.
3.5 inch diskettes cannot be inserted unless diskette is oriented correctly. This is as far as a disk can be inserted upside-down. The beveled corner of the diskette along with the fact that the diskette is not square, prohibit incorrect orientation.
Activity
Applying Universal Design – from learning to practice
Class Exercise
Each table or group is to list other examples of
Poka-Yoke (mistake proofing) devices
Brainstorming!!
Class Exercise
Mistake-Proof This:
A local countertop company makes and then loads the countertop pieces on to a truck and goes to the customer’s home and installs the countertop. More than 15% of all installation jobs have to be done with more than one trip to the customer’s home because one or more pieces of the countertop was not loaded on the truck.
Propose a change that will ensure all the material is loaded on the installation truck.
Class Exercise
Mistake-Proof This:
Mechanics in the instrument lab have to calibrate instruments in accordance with a procedure that has several steps. They sometimes inadvertently skip a step in the procedure.
Propose a change that will ensure no steps are skipped.
Class Exercise
Mistake-Proof This:
The far-sighted instructor in a rush to get to work on time often forgot and left his glasses at home.
Propose a change that will ensure the instructor does not forget his glasses.
References
The Principles of Universal Design‚ Version 2.0 (1997) by North Carolina State University (as cited in Preiser & Ostroff ‚ 2001)
Center for Universal Design (US) Home of the Principles of Universal Design, Exemplars of Universal Design, universal design history, the Design File, Center for Universal Design Newsline, publications, and more. http://www.design.ncsu.edu/
CAST (US) Home of Bobby, the web accessibility analysis tool, Universal Design in Learning and the National Center On Accessing the General Curriculum, and eProducts. http://www.cast.org
DO-IT: Disabilities, Opportunities, Internetworking, and Technology, http://www.washington.edu/doit/
Adaptive Environments Center (US) Home of the South Boston Waterfront Project, Designing for the 21st Century Conference, Access to Public Schools, New England ADA Technical Assistance Center, universal design education and consulting, Access to Design Professions, publications and more. http://www.adaptenv.org
Accessible Electronic & Information Technology: Legal Obligations of Higher Education and Section 508, Cynthia D. Waddell, J.D., 1999, http://athenpro.org/node/54
Next class
Next class: Universal Design Principle: Perceptible Sound
Readings: Erlandson, Chapter 7
