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©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 1 User interface design l Designing effective interfaces for software systems.

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Presentation on theme: "©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 1 User interface design l Designing effective interfaces for software systems."— Presentation transcript:

1 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 1 User interface design l Designing effective interfaces for software systems

2 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 2 Objectives l To suggest some general design principles for user interface design l To explain different interaction styles l To introduce styles of information presentation l To describe the user support which should be built-in to user interfaces l To introduce usability attributes and system approaches to system evaluation

3 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 3 Topics covered l 15.1 User interface design principles l 15.2 User interaction l 15.3 Information presentation l 15.4 User support l 15.5 Interface evaluation

4 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 4 The user interface l System users often judge a system by its interface rather than its functionality l A poorly designed interface can cause a user to make catastrophic errors l Poor user interface design is the reason why so many software systems are never used l Software engineers generally must do interface design

5 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 5 Importance of User Interface l “Most important part of any computer system” “Interface is the system for most users” l Increasingly important GUIs a big improvement over previous approaches Platforms (e.g. Mac/ Microsoft) have style guides 50% of code devoted to interface l Interface should “disappear” – users can focus on their task, not the interface l Biggest enemy of good interface design is time Galitz

6 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 6 Benefits of Good Design l Small improvements can be worth big $$$ Book e.g. if users work 1 sec slower on each of 4.8 million screens per year, need almost an extra person Book e.g.s Redesigns have improved productivity 20%, 25%, 40%, 50% … IBM - $1 invested in usability returns $10-$100 l Interface problems are treated as bugs Pressman - $1 fix during design, $10 fix during development, $100 fix after release l Big Improvements can establish new products, companies, markets … the browser was a UI idea – before it, search using gopher etc was tedious. AOL was successful because it was more user friendly than early leader CompuServe. Galitz

7 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 7 Graphical user interfaces l Most users of business systems interact with these systems through graphical interfaces although, in some cases, legacy text-based interfaces are still used

8 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 8 GUI characteristics Windows Icons Menus Pointing Graphics

9 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 9 GUI advantages l They are easy to learn and use. l The user may switch quickly from one task to another and can interact with several different applications. l Fast, full-screen interaction is possible with immediate access to anywhere on the screen

10 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 10 User-centred design l The aim of this chapter is to sensitise software engineers to key issues underlying the design rather than the implementation of user interfaces l User-centred design is an approach to UI design where the needs of the user are paramount and where the user is involved in the design process l UI design always involves the development of prototype interfaces

11 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 11 User interface design process

12 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide UI design principles l UI design must take account of the needs, experience and capabilities of the system users l Designers should be aware of people’s physical and mental limitations (e.g. limited short-term memory) and should recognise that people make mistakes l UI design principles underlie interface designs although not all principles are applicable to all designs

13 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 13 User interface design principles PrincipleDescription User FamiliarityInterface should use terms familiar to users ConsistencyComparable operations should be started the same way Minimal SurpriseUsers should never be surprised RecoverabilityUsers should be able to recover from their errors User GuidanceMeaningful feedback, context-sensitive help User DiversityShould provide for different types of user

14 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 14 Design principles l User familiarity The interface should be based on user-oriented terms and concepts rather than computer concepts. For example, an office system should use concepts such as letters, documents, folders etc. rather than directories, file identifiers, etc. l Consistency The system should display an appropriate level of consistency. Commands and menus should have the same format, command punctuation should be similar, etc. l Minimal surprise If a command operates in a known way, the user should be able to predict the operation of comparable commands

15 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 15 Design principles l Recoverability The system should provide some resilience to user errors and allow the user to recover from errors. This might include an undo facility, confirmation of destructive actions, 'soft' deletes, etc. l User guidance Some user guidance such as help systems, on-line manuals, etc. should be supplied l User diversity Interaction facilities for different types of user should be supported. For example, some users have seeing difficulties and so larger text should be available

16 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 16 Nielsen’s Ten Usability Heuristics l Visibility of system status l Match between system and the real world l User control and freedom l Consistency and standards l Error prevention l Recognition rather than recall l Flexibility and efficiency of use l Aesthetic and minimalist design l Help users recognize, diagnose, and recover from errors l Help and documentation Nielsen

17 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 17 Galitz’s Heuristics (Table 14.2) 1. Automate unwanted workload 2. Reduce uncertainty 3. Fuse data 4. Present new info with meaningful aid to interpretation 5. Use names that are conceptually related to functions 6. Group data in consistently meaningful ways to reduce search time 7. Limit data-driven tasks 8. Include in displays only info needed by user at a given time 9. Provide multiple coding of data where appropriate 10. Practice judicious redundancy Galitz

18 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 18 Galitz’s WWW Heuristics 1. Speak the user’s language 2. Be consistent 3. Minimize the user’s memory load 4. Build flexible and efficient systems 5. Design aesthetic and minimalist systems 6. Use chunking 7. Provide progressive levels of detail 8. Give navigational feedback 9. Don’t lie to the user Galitz

19 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide User-system interaction l Two problems must be addressed in interactive systems design How should information from the user be provided to the computer system? How should information from the computer system be presented to the user? l User interaction and information presentation may be integrated through a coherent framework such as a user interface metaphor

20 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 20 Interaction styles l Direct manipulation l Menu selection l Form fill-in l Command language l Natural language

21 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 21 Direct manipulation advantages l Users feel in control of the computer and are less likely to be intimidated by it l Fast and intuitive interaction l User learning time is relatively short l Users get immediate feedback on their actions so mistakes can be quickly detected and corrected

22 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 22 Direct manipulation problems l The creation of an appropriate information space model (metaphor) for real world tasks and objects can be very difficult l Given that users have a large information space, what facilities for navigating around that space should be provided? l Direct manipulation interfaces can be complex to program and make heavy demands on the computer system

23 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 23 Direct Manipulation Applications l Games l CAD systems l Files and Folders

24 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 24 Control panel interface

25 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 25 Menu systems l Users make a selection from a list of possibilities presented to them by the system l The selection may be made by pointing and clicking with a mouse, using cursor keys or by typing the name of the selection l May make use of simple-to-use terminals such as touchscreens

26 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 26 Advantages of menu systems l Users need not remember command names as they are always presented with a list of valid commands l Typing effort is minimal l User errors are trapped by the interface l Context-dependent help can be provided. The user’s context is indicated by the current menu selection

27 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 27 Problems with menu systems l Actions which involve logical conjunction (and) or disjunction (or) are awkward to represent l Menu systems are best suited to presenting a small number of choices. If there are many choices, some menu structuring facility must be used l Experienced users find menus slower than command language

28 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 28 Menu System Applications l Most general purpose systems

29 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 29 Form-based interface

30 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 30 Forms-based Systems Advantages l Simple data entry l Easy to learn

31 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 31 Forms-based Systems Disadvantages l Takes up a lot of screen space

32 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 32 Forms-based Systems Applications l Most systems involving significant data entry

33 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 33 Command interfaces l User types commands to give instructions to the system e.g. UNIX, DOS l Advantages: Powerful and flexible – good for experts Easy to process using compiler techniques Commands of arbitrary complexity can be created by command combination Concise interfaces requiring minimal typing can be created

34 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 34 Problems with command interfaces l Users have to learn and remember a command language. Command interfaces are therefore unsuitable for occasional users l Users make errors in command. An error detection and recovery system is required l System interaction is through a keyboard so typing ability is required

35 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 35 Command languages l Often preferred by experienced users because they allow for faster interaction with the system l Not suitable for casual or inexperienced users l May be provided as an alternative to menu commands (keyboard shortcuts). In some cases, a command language interface and a menu-based interface are supported at the same time

36 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 36 Natural language interfaces l The user types a command in a natural language. Generally, the vocabulary is limited and these systems are confined to specific application domains (e.g. timetable enquiries) l NL processing technology is now good enough to make these interfaces effective for casual users but experienced users find that they require too much typing

37 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 37 Multiple user interfaces

38 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide Information presentation l Information presentation is concerned with presenting system information to system users l The information may be presented directly (e.g. text in a word processor) or may be transformed in some way for presentation (e.g. in some graphical form) l The Model-View-Controller approach is a way of supporting multiple presentations of data

39 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 39 Information presentation

40 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 40 Model-view-controller

41 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 41 Information presentation l Static information Initialised at the beginning of a session. It does not change during the session May be either numeric or textual l Dynamic information Changes during a session and the changes must be communicated to the system user May be either numeric or textual

42 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 42 Information display factors l Is the user interested in precise information or data relationships? l How quickly do information values change? Must the change be indicated immediately? l Must the user take some action in response to a change? l Does the user need to interact with the displayed info via a direct manipulation interface? l Is the information textual or numeric? Are relative values important?

43 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 43 Alternative information presentations

44 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 44 Analogue vs. digital presentation l Digital presentation Compact - takes up little screen space Precise values can be communicated l Analogue presentation Easier to get an 'at a glance' impression of a value Possible to show relative values Easier to see exceptional data values

45 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 45 Dynamic information display

46 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 46 Displaying relative values

47 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 47 Textual highlighting

48 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 48 Data visualisation l Concerned with techniques for displaying large amounts of information l Visualisation can reveal relationships between entities and trends in the data

49 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide Colour displays l Colour adds an extra dimension to an interface and can help the user understand complex information structures l Can be used to highlight exceptional events l Common mistakes in the use of colour in interface design include over-use of colour in the display

50 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 50 Colour use guidelines l Don't use too many colours l Use colour coding to support user tasks l Allow users to control colour coding l Design for monochrome then add colour l Use colour coding consistently l Avoid colour pairings which clash l Use colour change to show status change l Be aware that colour displays are usually lower resolution

51 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide User support l User guidance covers all system facilities to support users including on-line help, error messages, manuals etc. l The user guidance system should be integrated with the user interface to help users when they need information about the system or when they make some kind of error l The help and message system should, if possible, be integrated

52 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 52 Help and message system

53 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 53 Error messages l Error message design is critically important. Poor error messages can mean that a user rejects rather than accepts a system l Messages should be polite, concise, consistent and constructive l The background and experience of users should be the determining factor in message design

54 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 54 Design factors in message wording

55 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 55 Design factors in message wording l Context l Experience l Skill Level l Style l Culture

56 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 56 Nurse input of a patient’s name

57 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 57 System and user-oriented error messages Error #27 Invalid patient id entered ? OK Cancel System-oriented error message

58 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 58 user-oriented error messages Patient J. Bates is not registered Click on Patients for a list of registered patients Click on Retry to re-input a patient name Click on Help for more information PatientsHelpRetryCancel User-oriented error message

59 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 59 Help system design l Help? means ‘help I want information” l Help! means “HELP. I'm in trouble” l Both of these requirements have to be taken into account in help system design l Different facilities in the help system may be required

60 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 60 Help information l Should not simply be an on-line manual l Screens or windows don't map well onto paper pages. l The dynamic characteristics of the display can improve information presentation. l People are not as good at reading screen as they are paper text. l Content should be prepared with help of application specialists l Content should not be too large – don’t overwhelm user

61 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 61 Help system use l Multiple entry points should be provided so that the user can get into the help system from different places. l Some indication of where the user is positioned in the help system is valuable. l Facilities should be provided to allow the user to navigate and traverse the help system. l Index, TOC, and Search should be provided

62 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 62 Entry points to a help system

63 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 63 Help system windows

64 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 64 Help on WWW l Easy to implement l Easy for users to use l Difficult to link to applications themselves users may need to make extra effort to get to help Help doesn’t know context where you needed help – cannot provide context sensitive help

65 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide User documentation l As well as on-line information, paper documentation should be supplied with a system l Documentation should be designed for a range of users from inexperienced to experienced l As well as manuals, other easy-to-use documentation such as a quick reference card may be provided

66 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 66 User document types

67 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 67 Document types l Functional description Brief description of what the system can do l Introductory manual Presents an informal introduction to the system l System reference manual Describes all system facilities in detail l System installation manual Describes how to install the system l System administrator’s manual Describes how to manage the system when it is in use

68 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide User interface evaluation l Some evaluation of a user interface design should be carried out to assess its suitability l Full scale evaluation is very expensive and impractical for most systems l Ideally, an interface should be evaluated against a usability specification. However, it is rare for such specifications to be produced

69 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 69 Usability attributes

70 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 70 Things to Test l Conformance with a requirement l Conformance with guidelines for good design l Identification of design problems l Ease of system learning l Retention of learning over time l Speed of task completion l Speed of need fulfillment l Error rates l Subjective user satisfaction Galitz

71 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 71 …System - User Interface Design Goals l 5 human factors central to community evaluation: 1.Time to learn 2.Speed of performance 3.Rate of errors by users 4.Retention over time 5.Subjective satisfaction l Trade-offs sometimes necessary l Test all design alternatives using mock-ups Galitz

72 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 72 Objective Measures of Usability l Effectiveness Speed of performance, # errors (against some standard) Tasks completeable by required pct of target users l Learnable Time to learn, amount of training and tech support needed (against some standard) Relearning time for intermittent users l Flexible l Subjective satisfaction Tiredness, discomfort, frustration, effort required, willingness/eagerness to use system Galitz

73 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 73 Simple evaluation techniques l Questionnaires for user feedback l Video recording of system use and subsequent tape evaluation. l Observation of users at work with system and “thinking aloud” about how they are trying to use system l Instrumentation of code to collect information about facility use and user errors. l The provision of a gripe button for on-line user feedback.

74 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 74 Kinds of Tests l Guidelines review l Heuristic evaluation (will be covered last due to extensive coverage) l Cognitive walkthrough l Think aloud evaluations l Usability test l Classic experiments l Focus groups Galitz

75 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 75 Elements of Discount Usability Engineering l Scenarios l Simplified Thinking Aloud l Heuristic Evaluation Nielsen

76 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 76 Scenarios l Take prototyping to extreme – reduce functionality AND number of features l Small, can afford to change frequently l Get quick and frequent feedback from users l Compatible with interface design methods Nielsen

77 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 77 Simplified Thinking Aloud l Bring in some users, give them tasks, have them think out loud l Fewer users in user testing Nielsen

78 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 78 Heuristic Evaluation l Context – part of iterative design l Goal – find usability problems l Who – small set of evaluators l How – study interface in detail, compare to small set of principles Nielsen

79 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 79 How to Conduct a Heuristic Evaluation l More than one evaluator to be effective. l Each evaluator inspects the interface by themselves l General heuristics may be supplemented l Results can be oral or written l Evaluator spends 1-2 hours with interface l Evaluator goes through interface > 1 time l Evaluators may follow typical usage scenarios l Interface can be paper Nielsen

80 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 80 Key points l Interface design should be user-centred. An interface should be logical and consistent and help users recover from errors l Interaction styles include direct manipulation, menu systems form fill-in, command languages and natural language l Graphical displays should be used to present trends and approximate values. Digital displays when precision is required l Colour should be used sparingly and consistently

81 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 81 Key points l Systems should provide on-line help. This should include “help, I’m in trouble” and “help, I want information” l Error messages should be positive rather than negative. l A range of different types of user documents should be provided l Ideally, a user interface should be evaluated against a usability specification

82 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 82 Norman’s Four Principles of Good Design l State and the action alternatives should be visible l Should be a good conceptual model with a consistent system image l Interface should include good mappings that reveal the relationships between stages l User should receive continuous feedback

83 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 83 Galitz’s Principles of User Interface Design l To follow … alphabetically (following book)

84 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 84 Aesthetically Pleasing l Meaningful contrast between screen elements l Create groupings l Align screen elements and groups l Provide 3 dimensional representation l Use color and graphics effectively and simply

85 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 85 Clarity l Visual elements l Functions l Metaphors l Words and text

86 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 86 Compatible l With the user l With the task and job l With the product (past systems)

87 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 87 Comprehensibility l User should easily be able to determine: What to look at What to do When to do it Where to do it Why to do it How to do it

88 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 88 Configurability l Users should be able to set preferences l Good Default Settings should be provided for non-tinkerers

89 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 89 Consistency l One of Shneiderman’s 8 golden rules for interface design l Similar components should Have similar look Operate similarly l Same action should always produce the same result l Function of elements should not change l Position of standard elements should not change l Same terminology used for same thing throughout l Standards and Guidelines increase the odds of consistency

90 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 90 Control l User should control the interaction Actions initiated by user Actions performed quickly Actions can be interrupted or stopped, and reversed l Context maintained is from the user’s perspective l More than one way to do things l Avoid modes l Configurable

91 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 91 Directness l Provide Direct Manipulation user selects an object, then directly performs an action on it The effect of action on an object should be immediately visible Available alternatives reduces memory load

92 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 92 Efficiency l Minimize eye and hand movements Make user actions flow from one to another Don’t switch users frequently from keyboard to mouse l Anticipate users wants and needs whenever possible

93 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 93 Familiarity l Use language and concepts familiar to the user l Keep the interface natural l Use real world metaphors

94 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 94 Flexibility l compatible with the user’s “skills, experience, habits, and preferences, and current conditions” l Danger: more flexibility increases complexity of system

95 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 95 Forgiveness l Prevent errors from occurring when possible l Tolerate and forgive common and unavoidable human errors l When an error occurs, provide constructive error messages l Protect against catastrophic errors

96 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 96 Predictability l User should be able to anticipate the flow of the task l Expectations should be fulfilled

97 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 97 Recovery l System should permit: Commands or actions to be abolished or reversed Immediate return to a certain point if difficulties arise l Users should never lose work due to: An error on their part Hardware, software, or communication problems

98 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 98 Responsiveness l Respond rapidly to user requests l Provide acknowledgement of user actions l Beginners need more / more informative feedback than experts do

99 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 99 Simplicity l Provide as simple an interface as possible Progressive disclosure Defaults Minimize screen alignment points Make common actions simple Provide consistency

100 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 100 Transparency l Allow user to focus on the task, not the interface Basic principle of direct manipulation Use user’s task vocabulary Design interface based on task analysis

101 ©Ian Sommerville 2000 Software Engineering, 6th edition. Chapter 15Slide 101 Trade-offs l Final Design will always represent a series of trade-offs l People’s requirements always take precedence over technical requirements


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