Lecture 7 Metrics and Ease of Use Fun, Function and Freedom CS 540 – Quantitative Software...

Lecture 7 Metrics and Ease of UseFun, Function and Freedom

CS 540 – Quantitative Software Engineering

Technical Metrics

Halstead McCabe Fan-in/Fan-out

Halstead -”software science”

Stresses syntactic units rather than LOC Model components:

• Operators - actions: +, -, *, /, if-then-else,…• Operands - data: variables and constants• 4 basic entities (used in a bunch of equations)

» n1 - # of different operators

» n2 - # of different operands

» N1 - total occurrences of operators

» N2 - total occurrences of operands

• Length of Program for Halstead: N = N1 + N2

Halstead use

Simple to calculate, no in-depth examination of structure Measure of overall quality of modules Helpful to program designers and to maintainers Substantial literature Requires coding of modules Automatic feedback to program developers Does not account for complexity of interfaces

McCabe’s Cyclomatic Complexity

Based on a directed graph showing control flow of program thereby showing the number of independent paths in the program

Cyclomatic Complexity, CV= e - n + p + 1 where:• e = # of edges• n = # of nodes• p = # of connected modules or code segments (1 for

main program and 1 for each procedure) ’10’ is the upper limit of McCabe complexity

McCabe use

Great for testing because it uncovers all linearly independent paths

Does not add more complexity to nested loops and in general does not consider context

Unlike Halstead does take into account control flow complexity, but not data flow

Useful for individual developer feedback and during maintenance

Fan In/Fan Out

Measures interactions, the number of modules that call a given module and number of modules called by a module.

Fan-in ≡ number of calling modules, where a concordance is needed to find the calling modules

Fan-out ≡ number of called modules High counts of fan-in/fan-out are undesirable Takes into account data driven programs but

underestimates complexity for programs/modules with little interactions

Interesting combined metric

Complexity ≈

[SLOC]|[( e - n + p + 1) [(Fan-in)(Fan-out)]2 ],

where CV = e - n + p + 1

Try it, you’ll like it.

Good user interfaces:

Improve end user productivity Require data entry only once/item Reduce training time Reduce errors Enhance end user acceptance “Don’t try to correct poor software design with

good documentation.” Yuhas

Ease of use checklist

Simple and natural dialog Speak the end user’s language Minimize human memory load Consistent Provide feedback Provide clearly-marked exits Provide shortcuts Provide helpful error messages

Northern Telecom NEC AT&T ROLMOverall Satisfaction 1 2 3 4

Most Frequently Recommended System 1 3 4 2

Training 1 4 3 2

Documentation 1 4 2 3

Attendant Operation 1 4 3 2

Installation / Cutover 1 2 3 4

System Management 1 2 4 3

User Operation 1 3 3 2

Hardware Reliability 1 3 3 4

Maintenance / Service 2 3 1 4

Troubleshooting 1 2 3 3

Systems Performed as Expected 1 4 3 4

DATAPRO PBX USER SATISFACTION Case STUDY

Human factors in interface design

Limited short-term memory• People can instantaneously remember about 7 items of information. If

you present more than this, they are more liable to make mistakes. People make mistakes

• When people make mistakes and systems go wrong, inappropriate alarms and messages can increase stress and hence the likelihood of more mistakes.

People are different• People have a wide range of physical capabilities. Designers should not

just design for their own capabilities. People have different interaction preferences

• Some like pictures, some like text.

Thanks to Ian Sommerville 2004 Software Engineering, 7th edition for this slide.

User interface importance

End users often judge a system by its interface rather than its functionality

A poorly designed interface can cause a user to abuse the system

Match the skills, experience and expectations of end users.

Unusable interfaces is the shortest path to shelf-ware.

Design concepts

Bend to familiarityThe 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.

Be ConsistentThe interfaces must be consistent from screen to screen.. Commands and

menus should be at the same place and have the same format,, etc. No surprises

A user should be able to infer the use of a command from similar ones.

More design concepts

RecoverabilityThe system should contain the effects of user errors and allow recovery

with minimal effort. This might include an undo facility, confirmation of destructive actions, 'soft' deletes, etc.

User guidanceSome user guidance such as help systems, on-line manuals, etc. should be

supplied. The design should strive for a ‘zero-training’ ideal. User diversity

Interaction facilities for different types of user should be supported. For example, both command lines and menus should be supported.

User Interface metaphor

How will user data be entered? How will computer generated information be

presented to the user? How can user interaction and information presentation be

integrated through a coherent framework?

Interaction styles

Interactionstyle

Main advantages Main disadvantages Applicationexamples

Directmanipulation

Fast and intuitiveinteractionEasy to learn

May be hard to implement.Only suitable where there is avisual metaphor for tasks andobjects.

Video gamesCAD systems

Menuselection

Avoids user errorLittle typing required

Slow for experienced users.Can become complex if manymenu options.

Most general-purpose systems

Form fill-in Simple data entryEasy to learnCheckable

Takes up a lot of screen space.Causes problems where useroptions do not match the formfields.

Stock control,Personal loanprocessing

Commandlanguage

Powerful and flexible Hard to learn.Poor error management.

Operating systems,Command andcontrol systems

Naturallanguage

Accessible to casualusersEasily extended

Requires more typing.Natural language understandingsystems are unreliable.

Informationretrieval systems

© Thanks to Ian Sommerville 2004 Software Engineering, 7th edition for this slide.

Information presentation


Information tobe displayed

Presentationsoftware

Display

Information presentation

Static information• Initialised at the beginning of a session. It does not change

during the session.

Dynamic information• Changes during a session and the changes must be

communicated to the end user.

Web sessions are one transaction with persistence remembered in a cookie.

Presentation methods

1

3

4 20 10 20

Dial with needle Pie chart Thermometer Horizontal bar


Error messages reflect your design

Messages must be polite, concise, consistent, complete, constructive. and helpful,

Error codes are ok iff they are coupled with a text message telling the user how to correct the error.

Avoid cryptic error messages.

Analysis techniques

Task analysis• Models the steps involved in completing a task.• Model frequency and sequences of tasks

Interviewing and questionnaires• Asks the users and their bosses about the work they do.

Ethnography• Observes the user at work

Model the system.• Extend the life of the prototype to system delivery• Calibrate the prototype with data from the actual system• Measure system performance under projected loads and

enhancements

User interface prototyping

The aim of prototyping is to allow users to gain direct experience with the interface.

Without such direct experience, it is impossible to judge the usability of an interface.

Prototyping may be a two-stage process:• Early in the process, paper prototypes may be used;• The design is then refined and increasingly sophisticated

automated prototypes are then developed.

Human computer interface design

HCI design is an iterative process involving close liaisons between users and designers.

The 3 core activities in this process are:• User analysis. Understand what the users will do with the

system;• System prototyping. Develop a series of prototypes for

experiment;• Interface evaluation. Experiment with these prototypes with

users.

© Ian Summerville 2004

The human computer interface

User interfaces should be designed to match the skills, experience and expectations of its anticipated users.

System users often judge a system by its interface rather than its functionality.

A poorly designed interface can cause a user to make catastrophic errors.

Poor user interface design is the reason why so many software systems are never used.


Poor Human Design

Human factors in interface design

Limited short-term memoryPeople make mistakes and when mistakes are made, inappropriate

alarms and messages increases stress the likelihood of more mistakes or the warnings are ignored.

People are differentPeople have a wide range of physical and mental abilities. Designers

must not just design for themselves. Test with naïve users.

People have different interaction preferencesSome like pictures, some like text and their preferences change as

they learn to use the software

The HCI design process

User analysis: Create the use cases Prototyping: Develop a series of HCI prototypes Interface evaluation: Experiment with these

prototypes with naïve users. Measure learning time, error rates, satisfaction…

Create encapsulation software object classes to separate screen layout details from data structure details.

Feedback the analysis of the experiments to the use cases that then influence the design


Human computer interface design principles

Principle Description

User familiarity The interface should use terms and concepts which are drawnfrom the experience of the people who will make most use of thesystem.

Consistency The interface should be consistent in that, wherever possible,comparable operations should be activated in the same way.

Minimal surprise Users should never be surprised by the behaviour of a system.

Recoverability The interface should include mechanisms to allow users torecover from errors.

User guidance The interface should provide meaningful feedback when errorsoccur and provide context-sensitive user help facilities.

User diversity The interface should provide appropriate interaction facilities fordifferent types of system user.


Ease of Use design objectives for HCI, ManualsForms, Printouts, Error Messages

Design Objective Technique

1. Be consistent Identical terminology, similar screen layout uniform escape routes, low response time variance

2. Provide shortcuts Direct menu access & function keys

3. Provide feedback Dashboards & warnings

4. Design tasks for completion.

Modularity

5. Fix errors efficiently Polite and comprehensive message, suggest changes

6. Easy action reversal ERASE commands, escape menus, update options

7. Local control Avoid frequent warnings & patronizing messages

8. Reduce short-term memory load

Simple displays, minimal use of windows, mnemonic coding, blank space, no clutter

9. Use surprise effectively Minimal highlighting, minimal input verification, avoid flashing and auditory messages

10. Keep user located Menu labels, graphic directories in help function, restrict menus to three levels

Why spend effort on the HCI?

Increased efficiency Improved productivity Reduced errors Reduced training - strive for game-like training Improved acceptance

Error messages reflect your design

Messages must be polite, concise, consistent, complete, constructive. and helpful,

Error codes are ok iff they are coupled with a text message telling the user how to correct the error.

Avoid cryptic error messages.

Definition –DoD US Military Standard for Human Engineering Design Criteria (1999)

• Achieve required performance by operator, control and maintenance personnel

• Minimize skill and personnel requirements and training time

• Achieve require reliability of personnel-equipment and software combinations

• Foster design standardization within and among systems

Definition -Humanistic

An interface is humane if it is responsive to human needs and considerate of human frailties• Boot up - that the user should not be kept waiting unnecessarily

is an obvious and humane design principle• Users should set the pace of interaction• Feedback status a la dashboards

:“A computer shall not harm your work or, through inaction, allow your work to come to harm” Vesonder (Asimov paraphrase)

A computer should not waste your time or require you to do more work than is strictly necessary• Data items must never be required to be keyed twice

Asimov’s Laws of Robotics

0. A robot may not injure humanity or, through inaction, allow humanity to come to harm.

1. A robot may not injure a human being or, through inaction, allow a human being to come to harm, except where that would conflict with the Zeroth Law.

2. A robot must obey orders given it by human beings except where such orders would conflict with the First Law.

3. A robot must protect its own existence as long as such protection does not conflict with the First or Second Law.

Design target: novice (N) or expert (E)

metricobjective

Time-to-learn

Response time

Error Rate Efficiency Reliability User Satisfaction

Effectiveness

1. Be consistent N N&E N&E N&E N&E N

2. Provide shortcuts E E E E E

3. Provide feedback N N&E N&E N&E

4. Design tasks for completion.

N N&E N&E

5. Fix errors efficiently

N&E N

6. Easy action reversal N N&E N&E N&E N&E N&E

7. Local control E E E E

8. Reduce short-term memory load

N N&E N&E N&E

9. Use surprise effectively

E N&E

10. Keep user located N N&E N&E N&E N&E N&E

Information display factors

Is the user interested in precise information or data relationships?

How quickly do information values change? Must the change be indicated immediately?

Must the user take some action in response to a change?

Is there a direct manipulation interface? Is the information textual or numeric? Are relative values

important?

Analogue or digital presentations?

Digital • Compact - takes up little screen space;

• Precise values can be communicated.

Analogue • Easier to get an 'at a glance' impression of a value;

• Possible to show relative values;

• Easier to see exceptional data values.

Displaying relative values

0 100 200 300 400 0 25 50 75 100

Pressure Temperature

Color Graphics

Color adds an extra dimension to an interface and can help the user understand complex information structures or confuse him.

Color can be used to highlight exceptional events. Common mistakes in the use of color in

interface design include:• The use of color to communicate meaning;

• Too much color

• Red/Green

As of 14 Nov

Color use guidelines

Limit the number of colors used and limit their use.

Use color change to show a change in system status.

Use color coding to support the task that users are trying to perform.

Be careful about color pairings.

Design factors in message wording

Factor Description

Context Wherever possible, the messages generated by the system should reflect the currentuser context. As far as is possible, the system should be aware of what the user is doingand should generate messages that are relevant to their current activity.

Experience As u sers become familiar with a s ystem they become irritated by long, ŌmeaningfulÕmessages. However, beginners find it difficult to understand short terse statements of aproblem. You should provide both types of message and allow the user to controlmessage conciseness.

Skill level Messages should be tailored to the userÕs skills as well as their experience. Messagesfor the different classes of user may be expressed in different ways depending on theterminology that is familiar to the reader.

Style Messages should be positive rather than negative. They should use the active ratherthan the passive mode of address. They should never be insulting or try to be funny.

Culture Wherever possible, the designer of messages should be familiar with the culture of thecountry where the system is sold. There are distinct cultural differences betweenEurope, Asia and America. A su itable message for one culture might be unacceptablein another.


Good and bad message design

Error #27

Invalid patient id

OK Cancel

System-oriented error messageUser-oriented error message

R. MacDonald is not a reg istered patient

Click on Patients for a list of patientsClick on Retry to re-input the patient’s nameClick on Help for more information

Patients Help Retry Cancel


Analysis techniques

Task analysis• Model each step involved in completing a task.• Model the sequences and repetition of tasks

Interviewing and questionnaires• Asks the users and their bosses, ‘what do you do?” and ‘who do

you work with?”

Ethnography• Observe the user at work

Model the system.• Extend the life of the prototype to system delivery• Calibrate the prototype with data from the actual system• Measure system performance under projected loads and

enhancements

User analysis

If you don’t understand what the users want to do with a system, you have no realistic prospect of designing an effective interface.

User analyses have to be described in terms that users and other designers can understand.

Scenarios where you describe typical episodes of use, are one way of describing these analyses.

Hierarchical task analysis

Retrieve picturesfrom remote

libraries

Discoverpossiblesources

Establishsearchterms

Search forpictures

Requestphotocopies

of found items

1 2 3 4.

Selectlibrary

Log in tocatalogue

Search forpictures

Modifysearch terms

Recordrelevantitems

3.1 3.2 3.3 3.4 3.5

Enter searchterms

Initiatesearch

Reviewresults

3.3.1 3.3.2 3.3.3

do 1, 2,3 until pictures found, 4

do 3.1, 3.2,3.3 until pictures found,3.4 if necessary, 3.5

do 3.3.1, 3.3.2, 3.3.3


Interviewing

Design semi-structured interviews using open-ended questions.

Passive Listening technique Users will say what they think essential; not just

what you thought of collecting. Group interviews or focus groups allow users to

clarify their thinking and move form the subconscious to the conscious..

Ethnography

Involves an external observer watching users at work and asking them in an unscripted way about their work.

Valuable because many user tasks are intuitive and they find these very difficult to describe and explain.

Also helps understand the role of social and organisational influences on work.

Check the ‘postems’ on their terminals

Ethnographic records

Air traffic control involves a number of control ‘suites’ where the suites controlling adjacent sectors of airspace are physically located next to each other. Flights in a sector are represented by paper strips that are fitted into wooden racks in an order that reflects their position in the sector. If there are not enough slots in the rack (i.e. when the airspace is very busy), controllers spread the strips out on the desk in front of the rack.

When we were observing controllers, we noticed that controllers regularly glanced at the strip racks in the adjacent sector. We pointed this out to them and asked them why they did this. They replied that, if the adjacent controller has strips on their desk, then this meant that they would have a lot of flights entering their sector. They therefore tried to increase the speed of aircraft in the sector to ‘clear space’ for the incoming aircraft.


Insights from ethnography

Air Traffic Controllers had to see all flights in a sector, so scrolling displays where flights disappeared off the top or bottom of the display were unacceptable.

The HCI must indicate how many flights were in adjacent sectors so that controllers can plan their work.


Perception of elapsed time

Jones, For Principles of Main Computer Dialog Computer Aided Design, Vol. 10, No. 3, pg 197, May 1978

Instantaneous Less than 1/3 sec workstation

Fast 1/3 to 1 second transaction

Pause 1 to 10 seconds www

Wait More than 10 seconds

HCI importance

End users often judge a system by its interface rather than its functionality

A poorly designed interface can cause a user to abuse the system

Match the skills, experience and expectations of end users.

Unusable interfaces is the shortest path to shelf-ware.

HCI Review

Govern software by HCI design. Prototype the HCI Choose an interaction styles that matches the problem:

• direct manipulation, • menu systems form fill-in, • command languages and • natural language.

Use graphics to show trends and approximate values. Use digital displays for precision.

Use color sparingly and consistently. Encapsulate

Usability attributes


Lecture 7 Metrics and Ease of Use Fun, Function and Freedom CS 540 – Quantitative Software...

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