GOMS Analysis & Web Site Usability

Melody Y. Ivory (UCB CS)SIMS 213, UI Design &

DevelopmentApril 15, 1999

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GOMS Analysis Outline GOMS at a glance Model Human Processor revisited Original GOMS (CMN-GOMS) Variants of GOMS GOMS in practice Summary

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GOMS at a glance Proposed by Card, Moran & Newell

in 1983– apply psychology to CS

»use user model (MHP) to predict performance of tasks in UI

task completion time, short-term memory requirements

– applicable to »user interface design and evaluation»training and documentation

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Model Human Processor (MHP) Revisited

Card, Moran & Newell (1983)– most influential model of

user interaction – 3 interacting subsystems

» cognitive, perceptual & motor» each with processor &

memory described by parameters

– e.g. capacity, cycle time» serial & parallel processing

Adapted from slide by Dan Glaser

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MHP Revisited Card, Moran & Newell

(1983)– principles of operation

» subsystem behavior under certain conditions

e.g. Fitts’s Law, Power Law of Practice

» ten total

Adapted from slide by Dan Glaser

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MHP Subsystems Perceptual processor

– sensory input (audio & visual)

– code info symbolically– output into audio & visual

image storage (WM buffers)

Adapted from slide by Dan Glaser

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MHP Subsystems Cognitive processor

– input from sensory buffers– access LTM to determine

response » previously stored info

– output response into WM

Adapted from slide by Dan Glaser

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MHP Subsystems Motor processor

– input response from WM– carry out response

Adapted from slide by Dan Glaser

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MHP Subsystem Interactions Input/output Processing

– serial action» pressing key in response to

light– parallel perception

» driving, reading signs & hearing

Adapted from slide by Dan Glaser

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MHP Parameters Based on empirical data

– word processing in the ‘70s Processors have

– cycle time () Memories have

– storage capacity ()– decay time of an item ()– info code type ()

» physical, acoustic, visual & semantic

Adapted from slide by Dan Glaser

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Perceptual Subsystem Parameters

Processor– cycle time () = 100 msec

Visual Image Store– storage capacity () = 17

letters– decay time of an item () =

200 msec– info code type () = physical

» physical properties of visual stimulus

e.g. intensity, color, curvature, length

Adapted from slide by Dan Glaser

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One Principle of Operation Power Law of Practice

– task time on the nth trial follows a power law

» Tn = T1 n-a, where a = .4» i.e., you get faster the more

times you do it!» applies to skilled behavior

(perceptual & motor)» does not apply to

knowledge acquisition or quality

Adapted from slide by Dan Glaser

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Original GOMS (CMN-GOMS) Card, Moran & Newell (1983) Engineering model of user

interaction– task analysis (“how to” knowledge)

»Goals - user’s intentions (tasks) e.g. delete a file, edit text, assist a customer

»Operators - actions to complete task cognitive, perceptual & motor (MHP) low-level (e.g. move the mouse to menu)

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CMN-GOMS Engineering model of user interaction

– task analysis (“how to” knowledge)»Methods - sequences of actions (operators)

based on error-free expert may be multiple methods for accomplishing same

goal– e.g. shortcut key or menu selection

»Selections - rules for choosing appropriate method

method predicted based on context– explicit task structure

»hierarchy of goals & sub-goals

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Text-Editing Example

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CMN-GOMS Analysis Analysis of explicit task structure

– add parameters for operators»approximations (MHP) or empirical data»single value or parameterized estimate

– predict user performance»execution time (count statements in task

structure)»short-term memory requirements (stacking

depth of task structure)– apply before user testing (reduce costs)

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Limitations of CMN-GOMS No directions for task analysis

– granularity (start & stop) Serial v.s. parallel perceptual

processing– contrary to MHP

Only one active goal Error-free expert performance

– no problem solving or evaluation»Norman’s Action Cycle

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Norman’s Action Cycle

Goals

EvaluationEvaluation of interpretations

Interpreting the perception

Perceiving the state of the world

ExecutionIntention to act

Sequence of actions

Execution of sequence of actions

The World

GOMS

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Variants of GOMS Keystroke-Level Model (KLM)

– simpler than CMN-GOMS»six keystroke-level primitive operators

K - press a key or button P - point with a mouse H - home hands D - draw a line segment M - mentally prepare to do an action R - system response time

»no selections»five heuristic rules (mental operators)

– still one goal activation

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Text-Editing Example (KLM)

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Variants of GOMS Natural GOMS Language (NGOMSL)

– more rigorous than CMN-GOMS»uses cognitive complexity theory (CCT)

user and system models– mapping between user’s goals & system model

user style rules (novice support)»task-analysis methodology» learning time predictions»flatten CMN-GOMS goal hierarchy

high-level notation (proceduralized actions) v.s. low-level operators

– still one goal activation

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Text-Editing Example (NGOMSL)

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Variants of GOMS Cognitive-Perceptual-Motor GOMS

(CPM-GOMS)– activation of several goals

»uses schedule chart (PERT chart) to represent operators & dependencies

»critical path method for predictions– no selections

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Text-Editing Ex. (CPM-GOMS)

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GOMS in Practice Mouse-driven text editor (KLM) CAD system (KLM) Television control system (NGOMSL) Minimalist documentation

(NGOMSL) Telephone assistance operator

workstation (CMP-GOMS)– saved about $2 million a year

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Summary GOMS in general

– The analysis of knowledge of how to do a task in terms of the components of goals, operators, methods & selection rules. (John & Kieras 94)

– CMN-GOMS, KLM, NGOMSL, CPM-GOMS Analysis entails

»task-analysis»parameterization of operators»predictions

execution time, learning time (NGOMSL), short-term memory requirements

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Web Site Usability Outline The Spool Study Major Implications

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The Spool Study Jared Spool et al. (96-97)

– Web Site Usability: A Designer’s Guide

Usability on the Web– shift from most E-Commerce studies

»converting clicks into sales– focus on people finding information to

make decisions (purchases)»sites that provide info to support sales

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Usability Testing of Web Sites Sites (Interfaces)

– 9 popular sites (products & info) Tasks

– information retrieval (4 types of tasks) Users

– familiar with Web browsers Not a formal usability study

– experiment design, number of users & experience, testing procedure?

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Sites 9 popular sites

– C|net - technology resources– Disney - original & redesigned– Edmund’s - car & truck resources– Fidelity - investments– Hewlett Packard (HP)– Inc. - original & redesigned– Olympics - 96 Olympics (expired)– Travelocity - travel resources– WebSaver - annuity information

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Tasks “Scavenger Hunt” Tasks

– retrieve information to answer questions– simple facts

» locating information e.g. Can you get a Honda Accord for under

$15,000?– comparison of facts

» locating two pieces of information plus a comparison

e.g. Which has better acceleration, the Jeep Cherokee or Toyota Land Cruiser?

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Tasks “Scavenger Hunt” Tasks

– judgment » locating information plus a decision

e.g. Do you think a used Ford F-10 is safe enough?

– comparison of judgment» locating multiple pieces of information

plus a decision e.g. Which convertible is the best deal for

under $20,000?

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Comparison of Sites How successful users were at

finding information?– Sites that were expected to do well

fared poorly and vice versa»Disney & C|net (graphically intense) »Edmund’s - mostly text

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Overall Site Usability Room for improvement

– finding information is an intensely frustrating experience for users»enormous time and effort to answer

simple questions (simple facts) even on small sites

»users give up without finding information

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Spool et al’sWeb Site Usability Issues

Classify each issue– information, navigation, graphic or

other design– very rudimentary

Total each category

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Ivory’s Web Site Usability Issues (Preliminary)

Web site usability information “findability”

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Web Site Usability Issues Navigation design

– number of links, location of links– within-page, wrapped, embedded,

image links Graphic design

– too much white space, unrelated or distracting graphics

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Web Site Usability Issues Information design

– no support for comparisons, poor readability

Other design– waiting for server

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Major Implications Graphic design neither helps nor

hurts– users may report as issue, but does

not correlate with users’ success Text links are vital

– downloading delays

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Major Implications Navigation and content are

inseparable– shell strategy leads to many generic

links Information retrieval is different than

surfing– implies different design approach

»surfing - need to attract users» information retrieval - help users find

information, more focused

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Major Implications Web sites aren’t like software

– software »success with product implies preference

– Web »success on site does not imply preference

content is important