NGOMSL Modeling
Psyc645 -- Week 10
Wayne D. Gray and Thomas Mayfield
George Mason University
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Next Week -- TUESDAY!!
 NGOMSL#1

Produce NGOMSL methods from the trace of the VCR (my trace not
yours)

Are no pops in NGOMSL. Verifies may be suitable replacements. Yours
to decide.
 NGOMSL#2

Create NGOMSL methods for the ManTel task

Should be one set of methods that can be applied to both interfaces and
to 1 or 2 queries

Obviously, not all methods need to be used in each analysis

Produce an NGOMSL trace for Dialog-Box 1 query and PopUp 2 queries
 Readings

Paul Green’s 1999 Human Factors Ergonomics Society conference
article
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Psyc645 -- Reprise of GOMS lecture 1

Levels of analysis based on Newell’s Time Scale of Human Action

Important to concept is that behavior at one level can be decomposed into behavior
at next lower level

Show what is added at various levels

Important to the argument that I am trying to make to convince you that analyses at
increasing lower levels entail decreasing degrees of generality (or increasing
degrees of specificity)

Programming VCR is a generally understood concept, but programming my VCR is
very different than programming George Lukas’ VCR. Setting Tom’s watch or Mary
Ann’s watch is very different than setting the clock in my car

Implications?

Behavior is not as unconstrained as we would like to believe. As we get more and
more specific in our description of behavior we find more and more constraints on
behavior.
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Psyc645 - Overview of GOMS lecture 2
 NGOMSL analyses

Unlike KLM, NGOMSL is a serious notation for discussing
interactive behavior at levels lower than the unit task

Not simply a tool for analyzing design but a vehicle for
representing low-level performance

Today we will work through several examples
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NGOMSL
 Natural Language GOMS

based on structured natural language notation and a procedure
for constructing them

models are in program form
 Control Structure: Hierarchical goal stack
 Serial or parallel: Serial
 Level of Analysis:

In principle, as necessary for your design question
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NGOMSL - why?
 More powerful than KLM. Much more useful for
analyzing large systems
 More built-in cognitive theory
 Provides predictions of operator sequence, execution
time, and time to learn the methods
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NGOMSL - Overall Approach
 Step 1: Perform goal/subgoal decomposition
 Step 2: Develop a method to accomplish each goal

List the actions/steps the user has to do (at as general and high-level as
possible for the current level of analysis)

Identify similar methods/collapse where appropriate
 Step 3: Add flow of control (decides)
 Step 4: Add verifies
 Step 5: Add perceptuals, etc.
 Step 6: Add mentals for retrieves, forgets, recalls
 Step 7: Add times for each step
 Step 8: Calculate total time
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NGOMSL - Example
 Car clock
 Presetting radio stations

simple

perverse
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NGOMSL--Car Radio example (1)
Provides predictions of methods and operators used to complete a task. If
you provide estimates of operator-duration, you can get predictions of
error-free expert performance time.
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NGOMSL-- Car Radio example (2)
 Goal/subgoal hierarchy
set clock
set
mode
turn knob
depress knob
(and keep it
depressed)
set hour
set
minute
press tune
“-” button
press tune “+”
button
release knob
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NGOMSL-- Car Radio example (3)
 Assumptions

User's hands are NOT on the buttons at the beginning.

Time to move hand & arm to time change level is estimated by
2ft "reach" from Barnes (1963): 410 msec.

D = device time, time for clock to move forward one number, =
500 msec (we estimate this on the next slide)

I am assuming a 12-hr clock. Seems good assumption for a car
clock.

Radio is off at beginning and end.

Begin knowing the time you want to set the clock to.
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NGOMSL-- Car Radio example (4)

D = device time, time for clock to move forward one number, =
500 msec
290
Visual Perception
perceive
info (X)
50
Cognitive Operators
50
verify
info (X)
initiate release
(X)
100
Right Hand
release
(X)
Minimum time required to perceive
clockTime=targetTime and to release the toggle lever
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NGOMSL-- Car Radio example (2)
•
(Duplicate slide)
•
Note that method for accomplishing the top level goal will have three
subgoals and one “release” physical operator
set clock
set
mode
turn knob
depress knob
(and keep it
depressed)
set hour
set
minute
press tune
“-” button
press tune “+”
button
release knob
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NGOMSL-- Car Radio example (5)
Method for goal: SET-CLOCK
Accomplish goal: set Mode to
Step 1 set Clock
Decide: If curHr°targetHr Then:
Accomplish goal: Change Hour
Step 2 display
Decide: If curMin°targetMin
Then: Accomplish goal: Change
Step 3 Minute display
stmt
time oper
0.1
op
time
tot
time
0.10
0.1
0.10
0.1
0.10
0.1
0.10
Step 4
Release "vol/on/off" knob
0.1 B
Step 5
Return with goal accomplished
0.1
0.10
assumptions
0.20
0.10
Need methods for each of these three subgoals
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NGOMSL-- Car Radio example (6)
Method for goal: set Mode to set Clock
stmt
op
tot
time op time time
0.1
0.10
Step 1 Recall "vol/on/off" is modeSwitch
0.1 M
Decide: If location of modeSwitch
is not known then Locate
Step 2 modeSwitch
0.1 M
Reach & Grasp "volume/on/off"
Step 3 knob
Step 4 Turn knob
0.1 R
0.1 T
Home thumb to "volume/on/off"
Step 5 knob
Step 6 Press and hold knob
Step 7 Return with goal accomplished
0.1 H
0.1 B
0.1
assumptions
retrieval from LTM or
equivalent (location and
0.50 0.60 perception of labels?) needed
For this example we assume
that location is NOT known. If
known then tot. time for this
step would be the stmt time,
1.20 1.30 0.10 sec
source: Barnes, 1963, reach =
0.48 0.58 410 msec, grasp = 070 msec
0.34 0.44 source: Barnes, 1963
400 msec seems too high
here, might use 100 msec for
0.40 0.50 button press instead.
0.10 0.20
0.10
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NGOMSL-- Car Radio example (7)
stmt
time
Method for goal: Change <time> display
Decide: If location of setTime
switch is not known then Locate
Step 1 setTime switch
Decide: If finger is NOT on
setTime switch then: Reach to
Step 2 setTime switch
Step 3
Step 4
Retrieve-from-LTM that <time>
= "+ or -"
Step 5
Determine current setting
Press and hold setTime switch
to <time>
Step 6
Step 7
Step 8
Hold until <curTime> =
<targetTime>, (500 x #digits)
Verify display <time>
Return with goal accomplished
oper
op
time
0.1
tot
time
0.10
assumptions
0.1 M
1.20
1.30 assume that loc is not known
0.1 R
0.41
0.51
0.50
retrieval from LTM or equivalent
(location and perception of labels?)
0.60 needed and takes - 500 msec
0.1 M
0.1 P
0.32
0.1 B
0.10
W
0.1 M
0.1
1.20
based upon cpm-goms analysis it should
take 420 msec for eye to move to clock
0.42 and for user to perceive the hour.
cpm-goms calc. is -250 msec, go with
0.20 KLM
System response time is 500
msec/digit, this is enough time for Ss
to perceive and initiate keyUp
response.
1.30
0.10
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NGOMSL-- Car Radio example (8)
time for SetClock goal
time for setMode goal
time for setHour from 1 to 10
time to setMin from 56 to 50
total
0.70
3.82
9.03
31.53
45.08
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NGOMSL-- PreSetting a station (1)
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NGOMSL-- PreSetting a station (2)
Goal hierarchy for optimal design
PreSET Network
Show
Set Mode to
Radio
Locate
Station
Set PreSET
etc
Select
AM/FM
seek &
identify
press & hold
preset button until
sound returns
Note differences between this and previous slide.
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NGOMSL-- PreSetting a station (3)
MODEL 1: SETTING A PRESET TO A NETWORK SHOW OPTIMAL DESIGN
Method for goal: preset network show
Step1: Decide: IF radio is not playing, THEN Accomplish goal:
set-mode-to-radio
Step2: Decide: IF show N.E. intended show, THEN Accomplish
goal: locate-station
Step3: Accomplish goal: set-preset
Step4: Return with goal accomplished
Method for goal: locate-station
Step1: Decide: IF band N.E. desired band, THEN Accomplish
goal: select AM/FM
Step2: Accomplish goal: seek&identify
Step3: Return with goal accomplished
Method for goal: select AM/FM
Step1: Reach to button for desired band
Step2: Press button for desired band
Step3: Verify band correct
Step4: Return with goal accomplished
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NGOMSL-- PreSetting a station (4)
Method for goal: seek&identify
Step1: Reach to right side of "Seek" button
Step2: Press "Seek" button
Step3: Decide: IF show N.E. intended show, THEN Goto 2
Step4: Verify show correct
Step5: Return with goal accomplished
Method for goal: set-preset
Step1: Reach to desired "Preset" button
Step2: Press and hold "Preset" button
Step3: Wait until sound returns
Step4: Remove finger from "Preset" button
Step5: Return with goal accomplished
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NGOMSL-- PreSetting perverse (1)
Goal hierarchy for perverse design
PreSET
Network Show
Locate
Station
Set Mode to
Radio
Set PreSET
etc
Select
AM/FM
Enter
seek
mode
Delete old
PreSET
Initiate
seek
seek &
identify
Exit
seek
MODE
Enter
delete
MODE
Enter PreSet
Button
Assign new
PreSET
Exit
delete
MODE
Enter
PreSet
MODE
Enter PreSet
Button
Exit
PreSet
MODE
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NGOMSL-- PreSetting perverse (2)
Radio for Perverse Preset Design
SEEK MODE
AM
FM
EJ
SEEK
–
ON
TUNE
ERASE PRESET
+
OFF
2:41
MEMORIZE PRESET
BASS
1
2
3
4
5
6
TREBLE
– + – +
BALANCE
L
R
FADE
L
R
PUSH CLOCK
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NGOMSL-- PreSetting perverse (3)
MODEL 2: SETT ING A PRESET T O A NET WORK SHOW PERVERSE DESIGN
Method for goal: preset network show (s/a optimal)
Step1: Decide: IF radio is not playing, THEN Accomplish goal:
set-mode-to-radio
Step2: Decide: IF show N.E. intended show, THEN Accomplish
goal: locate station
Step3: Accomplish goal: set-preset
Step4: Return with goal accomplished
Method for goal: locate-station (s/a optimal)
Step1: Decide: IF band N.E. desired band, THEN Accomplish
goal: select AM/FM
Step2: Accomplish goal: initiate-seek
Step3: Return with goal accomplished
Method for goal: select AM/FM (s/a optimal)
Step1: Put finger on button for desired band
Step2: Press button for desired band
Step3: Verify band correct
Step4: Return with goal accomplished
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NGOMSL-- PreSetting perverse (4)
Method for goal: initiate-seek
Step1: Accomplish goal setMode-SEEK
Step2: Accomplish goal: seek&identify
Step3: Accomplish goal: exit seek mode
Step4: Return with goal accomplished
Method for goal: setMode-<mode>
Step1: Reach for <mode> button
Step2: Press <mode> button
Step3: Verify <mode> correct
Step4: Return with goal accomplished
Method for goal: seek&identify (s/a optimal)
Step1: Reach to right side of "Seek" button
Step2: Press "Seek" button
Step3: Decide: IF show N.E. intended show, THEN Goto 2
Step4: Verify show correct
Step5: Return with goal accomplished
Method for goal: set-preset
Step1: Accomplish goal: delete-previous-preset
Step2: Accomplish goal: assign-new-preset
Step3: Return with goal accomplished
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NGOMSL-- PreSetting perverse (5)
Method for goal: delete-previous-preset
Step1: Accomplish goal: setMode-deletePreSet-on
(setMode-<mode>)
Step2: Accomplish goal: enter-preset-button
Step3: Accomplish-goal: setMode-deletePreSet-off
(setMode-<mode>)
Step4: Return with goal accomplished
Method for goal: assign-new-preset
Step1: Accomplish goal: setMode-preset-on (setMode<mode>)
Step2: Accomplish goal: enter-preset-button
Step3: Accomplish-goal: setMode-preset-off (setMode<mode>)
Step4: Return with goal accomplished
Method for goal: enter-preset-button
Step1: Reach to desired "Preset" button
Step2: Press "Preset" button
Step3: Return with goal accomplished
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NGOMSL--comparison of optimal and perverse designs
Optimal
number of different methods
Perverse
6
11
6
18
number of different steps
21
38
number of steps required
21
66
number of methods used
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Cost-of-Knowledge Characteristic
Function
 Card, S. K., Pirolli, P., & Mackinlay, J. D. (1994). The
cost-of-knowledge characteristic function: Display
evaluation for direct-walk dynamic information
visualizations. In B. Adelson, S. Dumais, & J. Olson
(Eds.), ACM CHI'94 Conference on Human Factors in
Computing Systems (Vol. 1, pp. 238-244). New York:
ACM Press.
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CENTURY-METHOD =
GOAL: DO-TASK
GOAL: GET-DATE
TURN-TO [MANUSCRIPT]
GET-DATE
GOAL: ACCESS-DAY-CALENDAR
GET-YEAR . . . if necessary
GOAL: SELECT-CENTURY (1700Хs)
POINT-TO (Century=1700-1790s))
=> Century-display
GET-YEAR . . . if necessary
GOAL: SELECT-DECADE (1710Хs)
POINT-TO (1710-1719))
=> Decade display
GET-YEAR . . . if necessary
GOAL: SELECT-YEAR: (1719)
POINT-TO (1719))
=> Year-display
GET-MONTH . . . if necessary
GOAL: SELECT-MONTH: (November)
POINT-TO (November))
=> Month-display
GET-DAY . . . if necessary
GOAL: SELECT-WEEK: [??]
POINT-TO [23]
=> Week-display
GET-DAY . . . if necessary
GOAL: SELECT-DAY: [23]
POINT-TO (23))
=> Day-display
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But,
 Card, Pirolli, & MacKinley did not do a complete
GOMS analysis
 Only did enough of an analysis to determine that the
“volume part” of the task was the repeated accesses
of menus for each component (century, decade, year,
month, week, day)
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Cost-of-Knowledge Function
 Small amounts of knowledge can be accessed
quickly with access costs increasing as amount of
knowledge accessed increases

≈6.9 s for 1 day

≈10.4 s for 7 days

≈14 s for 30 days

≈17.5 s for 365 days

≈21 s for 3,562 days

≈24.6 s for 36,525 days
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CD
= =Eliminate
need
“week”
to select
improve
“week”cycle
can do
time
= Eliminate
Spiral
as&is
BA
= SC
w/ 2 Calendar
s cycle time
(was 3.5)
that when select day from the month calendar
Design Improvements?
 Speed up system response time (SRT)

Time to access = 3.3 + 3.5 * Ncycles

Imply in text that it takes 1 s to point to item and 2.5 s for system to
bring up item
 Eliminate some of the cycles -- seem redundant

Get rid of the week cycle

Have users bring up month and directly select day from the month
calendar rather than first selecting week and then month
 Issue -
Implicit estimate of 2.5 s srt seems tediously long

Suggest need for a full NGOMSL analysis
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NGOMSL Methods for Spiral
Calendar
Top method and one subordinate
Method for goal: Set-Date
Step 1 Accomplish goal: get-date
Step 2 Accomplish goal: Access-day-calendar
Step 3 Return with goal accomplished
Total Time
Method for goal: get-date
stmt
op
time
op t/f time
0.1
0.1
0.1
0.1
0.1
Step
1 Decide: IF page not turned, then turn page
0.1 R
Step
Step
Step
2 Read Date
3 Home hand to keyboard
4 Return with goal accomplished
0.1 M
0.1 H
0.1
Total Time
tot time
0.10
0.10
0.10
0.10
0.40
assumptions
0.10
T
0.88
1.2
0.4
source: Barnes, 1963, reach = 410
msec, grasp = 070 msec, + 400 est for
flip. ** could analysis this further e.g.
0.98 CPM-GOMS
use M for estimate of time to read and
1.30 put into memory
0.50
0.10
2.98
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NGOMSL Methods (cont’)
Method for goal: access-day-calendar
Step 1 Decide: IF forgot date, then Read Date
Step 2 Accomplish Goal: Set-<timeUnit>
Decide: If current_day N.E. target_day, then
Step 3 GO TO Step 1
Step 4 Verify date
Step 5 Return with goal accomplished
Total Time
0.1
0.1 M
0.1
Method for goal: Set-<timeUnit>
Step 1 Point to <timeUnit>
Step 2 ClickOn <timeUnit>
Step 3 Wait for System Response
Step 4 Return with goal accomplished
0.1
0.1 P
0.1 BB
0.1 W
0.1
Total Time
0.1
0.1 M
0.1
F
1.20
1.20
1.10
0.20
1.00
0.10
0.10
0.10
0.10
1.30
0.10
1.80
0.10
1.20
0.30
1.10
0.10
2.80
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0.10 Method for goal: Set-Date
0.10
Step 1 Accomplish goal: get-date
0.10
Method for goal: get-date
0.98
Step 1 Decide: IF page not turned, then turn page
1.30
Step 2 Read Date
0.50
Step 3 Home hand to keyboard
0.10
Step 4 Return with goal accomplished
0.10
Step 2 Accomplish goal: Access-day-calendar
0.10
Method for goal: access-day-calendar
0.10
Step 1 Decide: IF forgot date, then Read Date
0.10
Step 2 Accomplish Goal: Set-<timeUnit>
0.10
Method for goal: Set-<timeUnit>
1.20
Step 1 Point to <timeUnit>
0.30
Step 2 ClickOn <timeUnit>
1.10
Step 3 Wait for System Response
0.10
Step 4 Return with goal accomplished
0.10
Step 3 Decide: If current_day N.E. target_day, then GO TO Step 1
0.10
Step 1 Decide: IF forgot date, then Read Date
0.10
Step 2 Accomplish Goal: Set-<timeUnit>
0.10
Method for goal: Set-<timeUnit>
1.20
Step 1 Point to <timeUnit>
0.30
Step 2 ClickOn <timeUnit>
1.10
Step 3 Wait for System Response
0.10
Step 4 Return with goal accomplished
0.10
Step 3 Decide: If current_day N.E. target_day, then GO TO Step 1
0.10
Step 1 Decide: IF forgot date, then Read Date
0.10
Step 2 Accomplish Goal: Set-<timeUnit>
0.10
Method for goal: Set-<timeUnit>
1.20
Step 1 Point to <timeUnit>
0.30
Step 2 ClickOn <timeUnit>
1.10
Step 3 Wait for System Response
0.10
Step 4 Return with goal accomplished
0.10
Step 3 Decide: If current_day N.E. target_day, then GO TO Step 1
1.30
Step 4 Verify date
0.10
Step 5 Return with goal accomplished
0.10
Step 3 Return with goal accomplished
How well does NGOMSL fit the data?
25
y = 0 .852 5x + 2 .431 5
2
R = 0.99
NGOMSL (se c)
20
15
10
5
0
0
5
10
15
20
25
30
Empirical Data (sec)
Pretty well by r2
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How well does NGOMSL fit the data?
em pirical data
card m odel
NGOMSL
30
Tim e in Seconds
25
20
15
10
5
0
1
2
3
4
5
6
Num ber of Cycles
Pretty well compared to the equation they
derived from the data
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How well does NGOMSL fit the data?
em pirical data
card m odel
NGOMSL
NGOMSL w/2.5 s srt
35
30
Tim e in Seconds
25
20
15
10
5
0
1
2
3
4
5
6
Num ber of Cycles
But not too well when I assume a 2.5 sec
system response time
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What Happened?
 Did the modeler cheat?
 Let’s examine the assumptions of the model
 Critical assumptions concern the time per cycle and
the elements that compose the cycle
 Card et al., empirically derived 3.5 s per cycle
 The NGOMSL model, which assumed a 1 s SRT,
predicted 3.1 s per cycle
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3.1 s
3.1 s
3.1 s
0.10 Method for goal: Set-Date
0.10
Step 1 Accomplish goal: get-date
0.10
Method for goal: get-date
0.98
Step 1 Decide: IF page not turned, then turn page
1.30
Step 2 Read Date
0.50
Step 3 Home hand to keyboard
0.10
Step 4 Return with goal accomplished
0.10
Step 2 Accomplish goal: Access-day-calendar
0.10
Method for goal: access-day-calendar
0.10
Step 1 Decide: IF forgot date, then Read Date
0.10
Step 2 Accomplish Goal: Set-<timeUnit>
0.10
Method for goal: Set-<timeUnit>
1.20
Step 1 Point to <timeUnit>
0.30
Step 2 ClickOn <timeUnit>
1.10
Step 3 Wait for System Response
0.10
Step 4 Return with goal accomplished
0.10
Step 3 Decide: If current_day N.E. target_day, then GO TO Step 1
0.10
Step 1 Decide: IF forgot date, then Read Date
0.10
Step 2 Accomplish Goal: Set-<timeUnit>
0.10
Method for goal: Set-<timeUnit>
1.20
Step 1 Point to <timeUnit>
0.30
Step 2 ClickOn <timeUnit>
1.10
Step 3 Wait for System Response
0.10
Step 4 Return with goal accomplished
0.10
Step 3 Decide: If current_day N.E. target_day, then GO TO Step 1
0.10
Step 1 Decide: IF forgot date, then Read Date
0.10
Step 2 Accomplish Goal: Set-<timeUnit>
0.10
Method for goal: Set-<timeUnit>
1.20
Step 1 Point to <timeUnit>
0.30
Step 2 ClickOn <timeUnit>
1.10
Step 3 Wait for System Response
0.10
Step 4 Return with goal accomplished
0.10
Step 3 Decide: If current_day N.E. target_day, then GO TO Step 1
1.30
Step 4 Verify date
0.10
Step 5 Return with goal accomplished
0.10
Step 3 Return with goal accomplished
Where is the problem?
 For NGOMSL to accept 2.5 s SRT means either

accepting a very bad fit, or

changing the model
 But, what can be changed?
 My guess is that they never measured the SRT of
their system
 In any event it is incredible that they assumed that all
of the cycle time not devoted to pointing was
attributable to SRT
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Two Morals to this Story?
 First

Even good researchers can be careless when they do not check
their assumptions against a model

(& yes, Card, Pirolli, and Mackinlay are VERY good researchers)
 Second
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Before investing lots of effort into jazzing up system response
time, do a profile analysis with a complete GOMS model
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Human Factors & Applied Cognitive Program
Next Week -- TUESDAY!!
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NGOMSL#1

Produce NGOMSL methods from the trace of the VCR (my trace not yours)
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Are no pops in NGOMSL. Verifies may be suitable replacements. Yours to decide.
NGOMSL#2

Create NGOMSL methods for the ManTel task
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Should be one set of methods that can be applied to both interfaces and to 1 or 2
queries
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Obviously, not all methods need to be used in each analysis
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Produce an NGOMSL trace for Dialog-Box 1 query and PopUp 2 queries
Readings

Paul Green’s 1999 Human Factors Ergonomics Society conference article
George Mason University
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Human Factors & Applied Cognitive Program
End of NGOMSL
George Mason University
48
Human Factors & Applied Cognitive Program