evaluation - wordpress.com...evaluation 0 tests usability and functionality of system 0 occurs in...
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Evaluation
0 Tests usability and functionality of system
0 Occurs in laboratory, field and/or in collaboration with users
0 Evaluates both design and implementation
0 Should be considered at all stages in the design life cycle
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Goals of Evaluation
Evaluates level of system functionality
Evaluates effect of interface on user
Identifies specific problems
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Evaluating Designs
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Cognitive Walkthrough
Proposed by Polson et al.
evaluates design on how well it supports user in learning task
usually performed by expert in cognitive psychology
expert ‘walks though’ design to identify possible problems using psychological principles
forms used to guide analysis
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Cognitive Walkthrough (ctd)
0 For each task walkthrough considers
what impact will interaction have on user?
what cognitive processes are required?
what learning problems may occur?
0 Analysis focuses on goals and knowledge: does the design lead the user to make the correct goals?
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Heuristic Evaluation
0 Proposed by Nielsen and Molich. usability criteria (heuristics) are identified
design examined by experts to see if these are violated
0 Examples of heuristics o system behaviour is predictable
o system behaviour is consistent
o feedback is provided
0 Heuristic evaluation `debugs' design. Debugging means identifying errors and fix them
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Review-based evaluation
0 Results from the written review used to support or refute (disapprove) parts of design.
0 Care needed to ensure results are transferable to new design.
0 Model-based evaluation
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Evaluating through user Participation
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Laboratory studies
0 Advantages: specialist equipment available
uninterrupted environment
0 Disadvantages: lack of context(environment)
difficult to observe several users cooperating
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Field Studies
0 Advantages: natural environment
context retained (though observation may alter it)
0 Disadvantages: distractions
noise
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Evaluating Implementations
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Experimental evaluation
controlled evaluation of interactive behaviour
evaluator chooses hypothesis(theory) to be tested
a number of experimental conditions are considered which differ only in the value of some controlled variable.
changes in behavioural measure are attributed to different conditions
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Experimental factors
Subjects
o who – representative, sufficient sample
Variables
o things to modify and measure
Hypothesis
o what you’d like to show
Experimental design
o how you are going to do it
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Variables
1. independent variable (IV) o characteristic changed to produce different conditions
o e.g. interface style, number of menu items
2. dependent variable (DV) characteristics measured in the experiment
e.g. time taken, number of errors.
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Hypothesis (thesis or theory)
0 prediction of outcome 0 framed in terms of IV and DV
e.g. “error rate will increase as font size decreases”
0 null hypothesis: 0 states no difference between conditions
e.g. null hyp. = “no change with font size”
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Experimental design
1. Within(inside) groups design o each subject performs experiment under each condition.
o less costly and less likely to suffer from user variation.
2. Between groups design o each subject performs under only one condition
o more users required
o variation can bias results.
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Analysis of data
0 Before you start to do any statistics: o look at data
o save original data
0 Choice of statistical technique depends on o type of data
o information required
0 Type of data o discrete - finite (fixed) number of values
o continuous - any value
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Analysis - types of test
0 Parametric assume normal distribution powerful
0 Non-parametric do not assume normal distribution less powerful more reliable
0 Likelihood table classify data by discrete attributes count number of data items in each group
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Analysis of data (cont.)
0 What information is required?
o is there a difference?
o how big is the difference?
o how accurate is the estimate?
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Experimental studies on groups
More difficult than single-user experiments
Problems with:
o subject groups
o choice of task
o data gathering
o analysis
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Subject groups
larger number of subjects more expensive
longer time to `settle down’ … even more variation!
difficult to timetable
so … often only three or four groups
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The task
1.Difficult task
2.Medium task
3.Easy task
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Data gathering
several video cameras + direct logging of application
Experimental Laboratory
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Field studies
Experiments ruled by group formation
Field studies more realistic:
work studied in context
real action is situated action
physical and social environment both crucial
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Observational Methods
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Think Aloud
0 user observed performing task
0 user asked to describe what s/he is doing and why, what s/he thinks is happening etc.
0 Advantages 0 simplicity - requires little expertise
0 can provide useful insight
0 can show how system is actually used
0 Disadvantages 0 Selective (careful)
0 act of describing may alter task performance
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Cooperative evaluation
0 variation on think aloud
0 user collaborates in evaluation
0 both user and evaluator can ask each other questions throughout
0 Additional advantages less constrained and easier to use
user is encouraged to criticize system
explanation possible
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Protocol analysis
paper and pencil – cheap, limited to writing speed
audio – good for think aloud, difficult to match with other protocols
video – accurate and realistic, needs special equipment computer logging – automatic, large amounts of data difficult to analyze
user notebooks – coarse and subjective, useful insights, good for longitudinal studies
0 Mixed use in practice.
0 audio/video transcription difficult and requires skill.
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automated analysis – EVA
0 Workplace project
0 Post task walkthrough 0 user reacts on action after the event
0 Advantages o analyst has time to focus on relevant incidents
o avoid unnecessary interruption of task
0 Disadvantages o lack of newness
o may be post-hoc interpretation of events
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post-task walkthroughs
0 transcript played back to participant for comment
immediately fresh in mind
delayed evaluator has time to identify questions
0 useful to identify reasons for actions and alternatives considered
0 necessary in cases where think aloud is not possible
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Query Techniques
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Interviews
0 analyst questions user on one-to -one basis usually based on prepared questions
0 informal, subjective and relatively cheap
0 Advantages o can be varied to suit context
o issues can be explored more fully
o can elicit user views and identify unanticipated problems
0 Disadvantages o very subjective
o time consuming
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Questionnaires
0 Set of fixed questions given to users
0 Advantages o quick and reaches large user group
o can be analyzed more rigorously
0 Disadvantages o less flexible
o less searching
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Questionnaires (ctd)
0 Need careful design o what information is required?
o how are answers to be analyzed?
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Questionnaire
0 Styles of question 1. General - establish background of user
2. Open-ended 0 ‘Can you suggest improvements to interface?’
3. Scalar 0 It is easy to recover from mistakes. Disagree 1 2 3 4 5 Agree
4. Multi-choice 0 How do you most often get help with the system? Choose one. online manual contextual help command prompt ask a colleague
5. Ranked – place a list of items in order
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Physiological methods
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eye tracking
0 head or desk mounted equipment tracks the position of the eye
0 eye movement reflects the amount of cognitive processing a display requires
0 measurements include 1. fixations: eye maintains stable position. Number and
duration indicate level of difficulty with display
2. saccades: rapid eye movement from one point of interest to another
3. scan paths: moving straight to a target with a short fixation at the target is optimal
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physiological measurements
0 emotional response linked to physical changes
0 these may help determine a user’s reaction to an interface
0 measurements include: 0 heart activity, including blood pressure, volume and pulse.
0 activity of sweat glands
0 electrical activity in muscle
0 electrical activity in brain
0 some difficulty in interpreting these physiological responses - more research needed
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Choosing an Evaluation Method
when in process: design vs. implementation
style of evaluation: laboratory vs. field
how objective: subjective vs. objective
type of measures: qualitative vs. quantitative
level of information: high level vs. low level
level of interference: obtrusive vs. unobtrusive
resources available: time, subjects, equipment, expertise