Task Analysis
CSCI 4800/6800
Feb 27, 2003
Goals of task analysis
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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
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Instantiate current tasks in new system, rather
than redesigning flow of work to achieve
desired higher-level function …
Lower-level view:
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Actions/operations:
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Pressing a key
Moving a mouse
Types of TA techniques:
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Focus on:
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Cognition
Practice of task
Logic of task
Goals, tasks, actions : terminology
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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
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Hierarchical Task Analysis (HTA)
Cognitive Task Analysis
Modeling “how to” knowledge
HTA: Hierarchical Task Analysis
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Graphical representation
Decomposition of high level task into
constituent subtasks, operations, plans
Uses structure chart notation
Structure chart notation
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activities ordered left to right
* indicates iteration
○ indicates selection
----- indicates absence of an action
Creating an HTA diagram:
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Start the analysis
Progress the analysis
Finalize the analysis
Starting the Analysis
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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
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Choose level of detail
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“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
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Check for consistency in decompositions and
numbering of tasks
Consult with user knowledgeable in task
domain
Cognitive Task Analysis
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Inform the design process through application of
cognitive theories
Some tasks, actions are cognitive – define these
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Examples:
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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
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MHP – Model Human Processor (Card ’83)
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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
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TKS – Task Knowledge Structures (Johnson
’92)
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Theory: Assumes that as people learn and perform
tasks, they develop knowledge structures
KAT – Knowledge Analysis of Tasks (Johnson
’92)
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Method: identify the elements of knowledge
represented in a task knowledge structure
Other techniques …
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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
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“how to do it” knowledge
Focuses on task to action mapping
GOMS (Goals, Operations, Methods, Selection
Rules) is most famous approach
The GOMS approach
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Three levels of granularity:
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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
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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.
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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
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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
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works well for simple tasks, unwieldy for larger
tasks
requires substantial time, skill, effort
results questioned by some
Representing task knowledge
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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...
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focus has shifted to less formal methods
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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
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Vocabulary for describing aspects of information
structures:
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Viscosity
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Delayed gratification
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effort required to meet goal.
Premature commitment
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resistance to change. How easy is it to make changes to some
aspects of the artifact?
user is forced to make choices too soon
Hidden dependencies
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information links that aren’t easily visible
ERMIA -
Entity Relationship Modeling for Information
Artifacts
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Uses ER notation
One for conceptual level
One for perceptual level
ERMIA – example: hidden
dependecy
Spreadsheet cell
1
uses
1
used by
M
M
Related cell
conceptual level
Spreadsheet cell
1
uses
M
Related cell
perceptual level
Highlights problem w/ spreadsheets – not visible which cells
use this cell, only which cells are used by this cell
Exercise
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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
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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
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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.