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© Simeon Keates 2009
Usability with ProjectLecture 14 – 30/10/09Dr. Simeon Keates
© Simeon Keates 2009
Exercise – Part 1
Last week you were asked to prepare your user trial protocols Today – put them into practice
Perform a pilot study of the usability of your web-site with at least 1 user
Remember – the principal aim is to “test the test” • (or “trial the trial” or “evaluate the evaluation”…)
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© Simeon Keates 2009
Exercise – Part 2
Prepare a progress presentation for the board for Friday Show that good progress is being made
Summarise:• The tasks performed • The data collected• Whether the user liked the site• Whether the user could use the site (e.g. complete the tasks)• What you think is working well in the design• What you think needs to be looked at more closely in the design• Any changes you would like to make to the site and protocol
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© Simeon Keates 2009
Exercise - Practicalities
Remember to print out copies of your protocol
Allow plenty of blank space for adding observation notes
Allocate one person to do the pre-session briefing and debrief
Allocate one person to be the facilitator (the person who directs the user)
The remaining members act as observers
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© Simeon Keates 2009
Cognitive models
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© Simeon Keates 2009
The Power Law of Practice
Tn = T1 n-α
α = 0.4, T1 = 60s, T2 = 45.5s (24% faster), T10 = 23.9s (60%faster)
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© Simeon Keates 2009
Cognitive modelling – Dealing with uncertainty
The Uncertainty Principle states that decision time T increases with uncertainty about the decision to be made:
T = Ic H
Where: H is the information-theoretic entropy of the decision;
Ic = 150 [0~157] ms/bit
For n equally probable alternatives (Hick’s Law) :
H = log2(n + 1)
More generally:
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€
H = pi log2( 1 pii∑ +1)
© Simeon Keates 2009
Cognitive modelling – The Model Human Processor
Time_taken = x τp + y τc + z τm
Where : x, y and z are integers
τp = time for perceptual processor
τc = time for cognitive processor
τm= time for (simple) motor function
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© Simeon Keates 2009
Motor skills – Positioning time
The time to perceive something includes the time for your eye to be looking at the right thing
Similarly, motor functions also involve a “time for location”
Common sense says that:• The further away something is, the longer it takes to reach it• The smaller a target is, the longer it takes to “hit” it
Also, human movement is a 2 stage process Stage 1 – gross (ballistic) movement• Covers most of the distance quickly, but not very accurately
Stage 2 – fine (homing) movement• Refine the position on to the target
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© Simeon Keates 2009
Motor skills – Fitts’ Law
A person wishes to hit this target:
We know that a correction cycle takes:
τp + τc + τm≈ 240 ms
And so n corrections takes n * 240 ms
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Startx0 x1 x2
S
D
© Simeon Keates 2009
Fitts’ Law
Now let xi be the remaining distance after the i-th correction
And let x0 (= D) be the starting point
We will assume that the relative accuracy of movement is constant, i.e.:
Where ε < 1 and is the constant error
On 1st cycle: x1 = ε x0 = ε D
On 2nd cycle: x2 = ε x1 = ε (ε D) = ε2 D
On n-th cycle: xn = εn D
Process stops when: εn D ≤ ½ S
Solving for n gives:
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€
x ix i−1
= ε
€
n = −log2(2D /S)
log2 ε
© Simeon Keates 2009
Fitts’ Law
From:
Total movement time, Tpos is given by:
This can be re-written as:
Where:
ε has been found to be ~ 0.7
Thus IM ≈ -240 / log2(0.7) = 63 ms/bit [27~122 ms/bit]
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€
n = −log2(2D /S)
log2 ε
€
Tpos = n(τ p + τ c + τ m )
€
Tpos = IM log2(2D /S)
€
IM =−(τ p + τ c + τ m )
log2 ε
Fitts’ Law
© Simeon Keates 2009
Fitts’ Law corrections
There are several modifications to Fitts’ Law
Fitt’s Law becomes less accurate for low values of log2(2D / S)
i.e. where the target is quite big compared with the distance
An example correction by Welford (1968):
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€
Tpos = IM log2(D S + 0.5)
© Simeon Keates 2009
Fitts’ Law – Implications for web-site design
Long, thin targets are not good• Small S value => longer acquisition times
Example of long, thin target:• Text-only hyperlinks• e.g. Heinz tomato ketchup
Better to include something large• e.g. an image of a ketchup bottle…
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© Simeon Keates 2009
Merging the models
One basic merged model is the Keystroke Level Model (KLM):
Texecute = TK + TP + TH + TD + TM + TR
Where TK = total time spent keystroking = nk tk (# * time per stroke)
• Time per stroke determined experimentally
TP = total time spent pointing (from Fitts’ Law)
• Assume, say, 1.1 s per pointing action
TH = total time spent homing (moving hands between devices)
• Assume 0.4 s per homing
TD = total time spent drawing = tD (nD, lD) (i.e. f(#, total length))
• Example: 0.9nD + 0.16lD
TM = total time to mentally prepare
• Assume 1.35 s per preparation
TR = total system response timePage 15
© Simeon Keates 2009
Using the KLM
[Note: M = mental prep, K = keyboard, P = pointing] Rule 0: Insert Ms in front of all Ks that are not part of argument strings
proper. Place Ms in front of all Ps that select commands Rule 1: If an operator following an M is fully anticipated in an operator
just previous to M, then delete the M (e.g. PMK -> PK) Rule 2: If a string of MKs belongs to a cognitive unit (e.g. name of a
command), then delete all Ms but the first one Rule 3: If a K is a redundant terminator (e.g. terminates a command
immediately following the terminator of its argument), then delete the M in front of it
Rule 4: If a K terminates a constant string (e.g. a command name), then delete the M in front of it, but if the K terminates a variable string (e.g. an argument string) then keep the M in front of it
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© Simeon Keates 2009
An more generic approach - GOMS
The user’s cognitive structure consists of: A set of Goals A set of Operators A set of Methods A set of Selection rules
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© Simeon Keates 2009
GOMS – a quick breakdown
Goals: Symbolic structures that define a state of affairs to be achieved• Examples: GOAL: EDIT-MANUSCRIPT or GOAL: MODIFY-TEXT• Goals can comprise sub-goals
Operators: Elementary perceptual, motor or cognitive acts whose execution is
necessary to change any aspect of the user’s mental state or to affect the task environment• Examples: GET-NEXT-PAGE or GET-NEXT-TASK
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© Simeon Keates 2009
GOMS – a quick breakdown
Methods: Procedures for accomplishing a goal – must be pre-learned at
performance time (i.e. user already knows them)• Contain sets of Operators
Selection rules: Rules for helping the user decide which method to use to accomplish the
goal• Example: if_such_and_such_is_true_then_use_method_M1_else_use_M2
To summarise: Several Operators make up a Method, and Selection rules are used to determine the best Method to reach the Goal
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© Simeon Keates 2009
Using models of interaction
Fundamentally, you need to perform a comprehensive task analysis
The models indicate suggested performance for each sub-task
Those models help you to predict the performance of the interface
This can be used:• In design: Estimate performance using standard parameters to optimise your
design• In usability trials: Estimate the performance and compare with actual
observed data – investigate significant discrepancies
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© Simeon Keates 2009
Exercise
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© Simeon Keates 2009
Exercise
On Wednesday(-ish) you performed a pilot study
Today, make any changes you identified to your usability protocol
Also, make any changes to your web-site based on the feedback that you obtained
Please mail your finalised protocols to Stina, Susanne and me
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