© Simeon Keates 2009 Usability with Project Lecture 14 – 30/10/09 Dr. Simeon Keates.

© Simeon Keates 2009

Usability with ProjectLecture 14 – 30/10/09Dr. Simeon Keates


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”…)


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


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


Cognitive models


The Power Law of Practice

Tn = T1 n-α

α = 0.4, T1 = 60s, T2 = 45.5s (24% faster), T10 = 23.9s (60%faster)


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:

€

H = pi log2( 1 pii∑ +1)


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


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


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

Startx0 x1 x2

S

D


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:

€

x ix i−1

= ε

€

n = −log2(2D /S)

log2 ε


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]

€

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


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):

€

Tpos = IM log2(D S + 0.5)


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…


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 time


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


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


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


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


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


Exercise


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

© Simeon Keates 2009 Usability with Project Lecture 14 – 30/10/09 Dr. Simeon Keates.

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Transcript of © Simeon Keates 2009 Usability with Project Lecture 14 – 30/10/09 Dr. Simeon Keates.