Constraints, Errors, and other Design Principles

1. Constraints, Errors, and other Design Principles

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

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

4. Affordances

5. Affordances

6. What’s wrongwith this picture?

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

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

9. 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)

10. 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

11. Perceptual Errors • 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

12. Motor Errors • Taxing the eye-hand coordination and motor skills • Awkward motor 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)

13. Cognitive Errors • 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 • e.g., where came from in a menu • Mental calculations and translations

14. Mistakes and Slips • Mistakes (hopefully avoidable) • Wrong intention • Incorrect mental model • Novice behavior • Slips (unavoidable) • Wrong execution • Skilled behavior

15. Moral … … slip happens

16. 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

17. 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”

18. 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 • Scroll wheel in PowerPoint

19. What to do? • As much as possible • Prevent mistakes • Allow recovery from slips

20. Error Prevention Guidelines • 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 • 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!

21. Examples

22. (Non) Understandable error messages

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

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

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

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

27. Predictability • I think that this action will do….

28. Familiarity • Does UI task leverage existing real-world or domain knowledge? • Really relevant to first impressions • Use of metaphors • Potential pitfalls

29. Consistency/Generalizability • Likeness in behavior between similar tasks/operations/situations/terminology • 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

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

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

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

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

34. Customizability • Ability of user to modify interface • By user - adaptability • Is this a good thing? • By system - adaptivity • Is this a good thing?

35. Robustness Principles • Supporting user in determining successful achievement and assessment of goals • Observability • Error Prevention & Recovery • Responsiveness

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

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

39. Responsiveness • Users perception of rate of communication with system (not always right) • Response time • 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

40. Responsiveness • Response to motor actions • 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

41. Responsiveness

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

43. Example • Most important usability principles for • Movie ticket kiosk • Instant messenger • Flight control

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

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