Constraints Errors and other Design Principles Design Principles

Constraints, Errors, and other Design Principles

Design Principles, Guidelines, Heuristics l l l Guidelines, practices to help designers No “cookbooks”, no universal checklists Are neither complete nor orthogonal – Can all be “broken”, often to satisfy another rule l Have underpinnings in psychology or experience or common sense l Understand the higher level principles that apply across situations, display types, etc. Implement the standards and guidelines l

We’ve already discussed many l l l l Affordances Visibility Conceptual models Mapping Feedback Web design – consistency, good links, etc. Visual – clarity, alignment, etc.

Affordances

Affordances

What’s wrong with this picture?

Constraints l Limitations on what can be done – – Physical - keys Semantic - menu graying Cultural - Colors Logical - When all above don’t apply

Without Constraints: Errors l l Make it hard for the user to make an error Don’t let the user do something that will lead to an error message

Why errors are important Errors are unavoidable To err is human Making mistakes is part of learning Designer’s responsibility Understand why errors occur Minimize likelihood Allow for recognition of error and graceful recovery (forward or backward)

Example Studies – Errors Happen! l 170 experienced UNIX users over 9 days – l Individual commands had error rates of 3 -50% Kraut et al, CHI ‘ 83 300 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 l Bad error recovery/help Mosteller & Ballas, Human Factors ‘ 89

Perceptual Errors l Result from insufficient or 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

Motor Errors l Taxing the eye-hand coordination and motor skills – – Awkward motor movements Highly similar motor sequences l – – – e. g. , double click, click Pressure for speed Require a high degree of hand-eye coordination Requiring special types of motor skills (type)

Cognitive Errors l Caused by taxing the memory and problem solving capabilities – – Tax recall memory Lack of or poor mnemonic aids Inconsistency Lack of context or status info l – e. g. , where came from in a menu Mental calculations and translations

Mistakes and Slips l Mistakes (hopefully avoidable) – – – l Wrong intention Incorrect mental model Novice behavior Slips (unavoidable) – – Wrong execution Skilled behavior

Moral … … slip happens

Types of Slips l 1. Capture error - Continue frequently done activity instead of intended one (similar starts) – l 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 l 3. Data driven error - Triggered by arrival of sensory info which intrudes into normal action – l Call to give someone a number, dial that number instead 4. Associative activation - Internal thoughts and associations trigger action – Phone rings, yell “come in”

Types of Slips l 5. Loss of activation - Forgetting goal in middle of sequence of actions – l 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 Scroll wheel in Power. Point

What to do? l As much as possible – – Prevent mistakes Allow recovery from slips

Error Prevention Guidelines l l l Eliminate modes or provide visible cues for modes - no invisible modes Use consistent representation (color, style) Maximize recognition, minimize recall Design non-similar motor sequences for commands Minimize need for typing Provide reasonableness checks on input data – l l l Did you really mean to order 5000? 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!

Examples

(Non) Understandable error messages

Error Recovery l Ability to take corrective action upon recognizing error – Forward recovery l – Backward recovery l – Undo previous error(s) Abort operation underway l l Ability to fix when we can’t undo Only makes sense if is a slow operation Encourages experimentation (hence learnability) by reducing cost of making mistakes

Implications for design l Scenarios should include errors !!! – l l Don’t forget this in your project! *hint* Distinguish between skilled and learner errors Uncover errors in the existing systems

Other Usability Principles l Categories – Learnability l – Flexibility l – l Support for multiple ways of doing tasks Robustness l l Support for learning for users of all levels Support for recovery Always think about exceptions, suitability Also good for prioritizing usability evaluations

1. Learnability Principles l Ease with which new users can begin effective interaction and achieve maximal performance – – Predictability Familiarity (Metaphor) Generalizability Consistency

Predictability l I think that this action will do….

Familiarity l Does UI task leverage existing realworld or domain knowledge? – Really relevant to first impressions – Use of metaphors l Potential pitfalls

Consistency/Generalizability l Likeness in behavior between similar tasks/operations/situations/terminology – l l Dialogue boxes always have yes/no/cancel buttons Avoid special cases and special rules Generalizability: take your knowledge of one system, and apply it to another

(In)Consistency Example Drag a file icon to: Folder on same physical disk Result: File is moved to folder Folder on another physical disk File is copied there Different disk File is copied there Trash can File is discarded

2. Flexibility Principles l Multiplicity of ways that users and system exchange information – – – Multi-threading: Allowing user to perform more than one task at a time Substitutivity Customizability

Substitutivity l Flexibility in details of operations – – Allow user to choose suitable interaction methods Allow different ways to l – perform actions, specify data, configure Allow different ways of presenting output l to suit task & user

Substitutivity l Equivalent values can be substituted for each other – – Point at spreadsheet cell vs enter name Give temperature via slider or by typing

Customizability l Ability of user to modify interface – By user - adaptability l – Is this a good thing? By system - adaptivity l Is this a good thing?

Robustness Principles l Supporting user in determining successful achievement and assessment of goals – – – Observability Error Prevention & Recovery Responsiveness

Observability l Can user determine internal state of system from what he/she perceives? – Browsability l l l – Reachability l – Navigate through observable states Persistence l – Explore current state (without changing it) Reduces memory load But don’t overwhelm user with information either How long does observable state persist? Observability also aids learnability

Observability - Scroll Bar l l Scroll bar size indicates % in view - but does not indicate absolute sizes. Can add other info, such as – Page 5 of 12

Responsiveness l l Users perception of rate of communication with system (not always right) Response time – l l Time for system to respond in some way to user action(s) Response OK if matches user expectations Once user enjoys fast response, is hard to go back to slower response – Dial-up versus DSL or Cable Modem

Responsiveness l Response to motor actions – – l Keyboarding, mouse movement - less than 100 msec. Rich Human Factors literature on this Consistency is important - experimental result – – Users preferred longer but more consistent response time Times that differed 10% to 20% were seen as same

Responsiveness

User experience goals – – – – Satisfying Fun Entertaining Enjoyable Helpful Motivating Aesthetically pleasing Motivating – – Rewarding Emotionally fulfilling Support creativity …and more

Example l Most important usability principles for – – – Movie ticket kiosk Instant messenger Flight control

Styleguides l Codify many of these principles for a particular look and feel – l Mac OS, Windows, Palm, Blackberry Developed in concert with toolkit, but go beyond toolkit

Your project l l l Which principles or guidelines are most important for your project? What does that principle mean for your design? How might you measure success?