Task Analysis

CSCI 4800/6800

Feb 27, 2003

Goals of task analysis

Elicit descriptions of what people do Represent those descriptions Predict difficulties, performance Measure learnability, transfer of knowledge

between systems Evaluate systems against usability and/or

functional requirements

Possible problem with TA

Instantiate current tasks in new system, rather than redesigning flow of work to achieve desired higher-level function …

Lower-level view:

Actions/operations:– Pressing a key– Moving a mouse

Types of TA techniques:

Focus on:– Cognition– Practice of task– Logic of task

Goals, tasks, actions : terminology

Goal = external task, such as producing a letter Device = method, tool, or technique appropriate

for achieving goals Tasks = activities necessary to achive goals

using a device Subtasks = components of tasks Actions = simple tasks w/ no control structure Method = plan = number of tasks or actions

linked into a sequence

Types of Task Analysis

Hierarchical Task Analysis (HTA) Cognitive Task Analysis Modeling “how to” knowledge

HTA: Hierarchical Task Analysis

Graphical representation Decomposition of high level task into

constituent subtasks, operations, plans Uses structure chart notation

Structure chart notation

activities ordered left to right * indicates iteration ○ indicates selection ----- indicates absence of an action

Creating an HTA diagram:

Start the analysis Progress the analysis Finalize the analysis

Starting the Analysis

Define the area of work / main task Break down main task into 4-8 subtasks;

specify subtasks in terms of objectives Draw subtasks as layered plans

Progressing the analysis

Choose level of detail– “click mouse” v. “delete block of text”

Choose depth-first, breadth-first, or combo Use hierarchical numbering convention

1, 2, 3, then 1.1, 1.2, 1.3 and so on ...

Finalizing the analysis

Check for consistency in decompositions and numbering of tasks

Consult with user knowledgeable in task domain

Cognitive Task Analysis

Inform the design process through application of cognitive theories

Some tasks, actions are cognitive – define these– Examples:

decide which button to press recall previously stored knowledge from memory compare two objects

Model the internal representation and processing that occurs for the purpose of designing tasks that can be undertaken more effectively by humans

Techniques for CTA

MHP – Model Human Processor (Card ’83)– Psychological model of humans as three interacting

systems: perceptual, motor, cognitive. Each system has its own memory and processor.

GOMS – Goals, Operators, Methods, and Selection Rules, and related NGOMSL(Kieras, ’88)

CCT – Cognitive Complexity Theory

Techniques for CTA

TKS – Task Knowledge Structures (Johnson ’92)– Theory: Assumes that as people learn and perform

tasks, they develop knowledge structures

KAT – Knowledge Analysis of Tasks (Johnson ’92)– Method: identify the elements of knowledge

represented in a task knowledge structure

Other techniques …

Focus on learnability (Task-Action Grammars) Focus on mappings from external tasks to

internal tasks (External Task Internal Task, Yoked State Spaces)

Modeling Procedural Knowledge

“how to do it” knowledge Focuses on task to action mapping GOMS (Goals, Operations, Methods, Selection

Rules) is most famous approach

The GOMS approach

Three levels of granularity:– GOMS model (general approach fo accomplishing

set of tasks)– unit task level : breaks users’ tasks into unit tasks,

then estimates the time it takes for user to perform these

– keystroke level: describes and predicts time it takes to perform a task by specifying the keystrokes needed.

Exercise w/ GOMS

In a word processor, a single word can be selected by double-clicking the cursor in the middle of the word. If the user wishes to select an arbitrary string of characters, the characters must be highlighted by dragging the mouse over the text with the mouse button held down. To cut the text out of a document, the user must first highlight the text and then issue the cut command.

– Write an NGOMSL description for the goal of ‘cut out text’.– Write out the selection rules for this example– Write out the method for selecting a word and for selecting an

arbitrary string of characters

Uses of GOMS models

to predict quality of existing system or prototype

to check of consistency of methods (similar goals are achieved by similar methods)

to check that most frequent goals are achieved by relatively quick methods

as a quantitative evaluation technique to choose between alternative designs

Problems with GOMS

works well for simple tasks, unwieldy for larger tasks

requires substantial time, skill, effort results questioned by some

Representing task knowledge

Important to understand what people already know about the specific task and this class of tasks

Now: focus on goal-to-task mapping “KAT” – knowledge analysis of tasks, looks at

this

More recently...

focus has shifted to less formal methods – claims analysis[Carroll & Kellogg, 89]– Scenario analysis[Young & Barnard, 91; Carey 91]– Cognitive walkthroughs[Lewis, 90]– Design rationale [Carroll and Moran 91; Maclean 91]– “cognitive dimensions” framework [Green 89]

Cognitive Dimensions

Vocabulary for describing aspects of information structures:

– Viscosity resistance to change. How easy is it to make changes to some

aspects of the artifact?– Delayed gratification

effort required to meet goal. – Premature commitment

user is forced to make choices too soon– Hidden dependencies

information links that aren’t easily visible

ERMIA --

Entity Relationship Modeling for Information Artifacts– Uses ER notation– One for conceptual level– One for perceptual level

ERMIA – example: hidden dependecy

Spreadsheet cell

Related cellRelated cell

Spreadsheet cell

M

1

M

used by1uses

1usesM

conceptual level perceptual levelHighlights problem w/ spreadsheets – not visible which cells use this cell, only which cells are used by this cell

Exercise

Draw an ERMIA diagram for the case of Word ‘style sheets”, based on the following description: – A style is a collection of character and paragraph

formats that can be applied to paragraphs or documents as required. A style can be based on another style. For example, the styles ‘List’ and ‘Quotation’ may be based on the style ‘Normal’. Changing style Normal will change all styles based upon it. The style display shows only the style that it is based upon.

Summary

Task analysis describes behaviors at three levels: goals, tasks, actions

Tasks are usually viewed in terms of a hierarchical decomposition of tasks into subtasks.

HTA and related techniques focus on what actually happens, rather than on what should happen.

Summary, continued

Cognitive task analysis techniques aim to describe some aspect of the cognitive characteristics of users’ tasks.

Some methods (such as GOMS) concentrate on users’ procedural or “how-to-do-it” knowledge.

Other methods focus on task knowledge. Many techniques difficult to use / don’t scale

well.