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ITM 734Introduction to Human Factors in Information Systems

Cindy Corritorecindycc@gmail.com

This material has been developed by Georgia Tech HCI faculty, and continues to evolve.

Simple Human Performance Models: Predictive Evaluation with Hick’s Law, Fitt’s Law, Power Law of Practice, Keystroke-Level Model

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Simple User Models

• Idea: If we can build a model of how a user works, then we can predict how s/he will interact with the interface Predictive model predictive

evaluation

• No mock-ups or prototypes!

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Two Types of User Modeling

• Cognitive – human as interperter/predictor – based on Model Human Processor (MHP) Key-stroke Level Model

– Low-level, simple GOMS (and similar) Models

– Higher-level (Goals, Operations, Methods, Selections)– Not discussed here

• Stimulus-Response Practice law Hick’s law Fitt’s law

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Keystroke-Level Model (KSLM)

• KSLM - developed by Card, Moran & Newell, see their book* and CACM* The Psychology of Human-Computer Interaction, Card,

Moran and Newell, Erlbaum, 1983

• Skilled users performing routine tasks• Assigns times to basic human operations -

experimentally verified• Based on MHP - Model Human Processor and

GOMS• Focuses on very low level actions

Assumes no high level thinking during action

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KSLM Accounts for

• Keystroking TK

• Mouse Button press TB

• Pointing (typically with mouse) TP

• Hand movement betweenkeyboard and mouse TH

• Drawing straight line segments TD

• “Mental preparation” for an action TM – how measure?

(fast recall)

• System Response time TR – ignore (fast)

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Using KSLM - Step One

• Decompose task into sequence of operations - K, B, P, H, D (no M operators yet; R can be used always or not at all) Typically system response time appears

instantaneous, so can be ignored

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Step One Example : MS Word Find Command

• Use Find Command to locate a six character word H (Home on mouse) P (Edit) B (click on mouse button - press/release) P (Find) B (click on mouse button) H (Home on keyboard) 6K (Type six characters into Find dialogue box) K (Return key on dialogue box starts the find)

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Using KSLM - Step Two

• Place M (mental prep) operators- In front of all K’s that are NOT part of

argument strings (ie, not part of text or numbers)

- In front of all P’s that select commands (not arguments)

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Step Two Example : MSoft Word Find Command

H (Home on mouse)MP (Edit)B (click on mouse button)MP (Find)B (click on mouse button)H (Home on keyboard)6K (Type six characters)MK (Return key on dialogue

box starts the find)

Rule 0b: Pselects command

Rule 0b: Pselects command

Rule 0a: Kis argument

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Using KSLM - Step 3

Remove M’s according to heuristic rules (Rules relate to chunking of actions)Rule 1. If action is anticipated by prior operation – it is a

chunk action– change PMK to PK (point and then click is a chunk)

Rule 2. If a string of MKs is a single cognitive unit (such as a command name), delete all MKs except the first

– MKMKMK -> MKKK (same as M3K) (again, it is a chunk)Rule 3. If it is a redundant terminator, such as )) at end of

something, then remove MRule 4. If the K terminates a constant string, such as

command word (such as return after typing in command), then delete M

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Step 3 Example: MS Word Find Command

Rule 4 Keep M

Rule 1 delete MH anticipates P

Rule 1 delete MH anticipates P

H (Home on mouse)MP (Edit)B (click on mouse button)MP (Find)B (click on mouse button)H (Home on keyboard)6K (Type six characters)MK (Return key on dialogue

box starts the find)

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Using KSLM - Step 4

• Plug in real numbers from experiments K: .08 sec for best typists, .28 average,

1.2 if unfamiliar with keyboard B: down or up - 0.1 secs; click - 0.2 secs P: 1.1 secs H: 0.4 secs M: 1.35 secs R: depends on system; often negligible

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Step 4 Example : MS Word Find Command

H (Home on mouse)P (Edit)B (click on mouse button - press/release)P (Find)B (click on mouse button)H (Home on keyboard)6K (Type six characters into Find dialogue box)MK (Return key on dialogue box starts the find)• Timings

H = 0.40, P = 1.10, B = 0.20, M = 1.35, K = 0.28 2H, 2P, 2B, 1M, 6K

• Predicted time = 6.43 secs

http://www.syntagm.co.uk/design/klmcalc.shtml

- website with KSLM calculator

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Power law of practice

• The logarithm of the reaction time for a particular task decreases linearly with the logarithm of the number of practice trials taken Time to perform a task based on

practice trials• Performance improves based on a

“power law of practice” That is, practice improves performance

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Power law of practice

• Tn = T1n-a

Tn time to perform a task after n trials T1 time to perform a task on first trial n number of trials (practice time) a is about .4, between .2 and .6 For learning skills - describes learning curve

– Typing speed improvement– Learning to use mouse– Pushing buttons in response to stimuli– NOT learning

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Uses for Power Law of Practice

• Use measured time T1 on trial 1 to predict whether time with practice will meet usability criteria, after a reasonable number of trials How many trials are reasonable?

• Predict how many practices will be needed for user to meet usability criteria Determine if usabiltiy criteria is realistic

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Hick’s law

• Decision time to choose among n equally likely alternatives – choice reaction time T = Ic log2(n+1) where

T is decision time

Ic ~ 150 msec (constant)

n is number of alternatives

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Uses for Hick’s Law

• Menu selection• Which will be faster as way to choose from

64 choices? Go figure: Single menu of 64 items Two-level menu of 8 choices at each level Two-level menu of 4 and then 16 choices Two-level menu of 16 and then 4 choices Three-level menu of 4 choices at each level Binary menu with 6 levels

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Fitts’ Law

• Models movement times for selection (reaching) tasks in one dimension

• Basic idea: Movement time for a selection task Increases as distance to target increases Decreases as size of target increases

• Function of distance and width (of target)

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Fitts model

MT = a +b log2(d/w +1)

• MT is average time taken to complete the movement

• a and b are constants and can be determined by fitting a straight line to measured data.

• d is the distance from the starting point to the center of the target.

• w is the width of the target measured along the axis of motion.

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Exact Equation

• Run empirical tests to determine k1 and k2

• Will get different ones for different input devices and device uses

MT

log2(d/w + 1.0)

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Uses for Fitt’s Law

• Menu item size• Icon size• Scroll bar target size and placement

Up / down scroll arrows together or at top and bottom of scroll bar

• Pie menus

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Cognitive models - many flavors

More complex than KSLMHierarchical

GOMS - Goals, Operators, Methods, SelectorsCCT - Cognitive Complexity Theory

LinguisticTAG - Task Action GrammarCLG - Command Language Grammar

Cognitive architecturesSOAR, ACT