Task Analysis


Task Analysis

Uses of Task Analysis

Goals, tasks and actions

the state that the user wishes to achieve
external task
an activity required to achieve a goal
internal task
a task which does not involve problem solving or control structure
simple task

Hierarchical Task Analysis

Task decomposition approach

HTA Structure Chart Notation

Structure Chart Notation

Structure Chart Notation

HTA Structure Charts

Tea-making Structure Chart

Tea-making Structure Chart

HTA Textual Notation

0. make tea
   1. boil water
      1.1 fill kettle
      1.2 light stove
      1.3 put kettle on stove
      1.4 wait
      1.5 turn off stove
   2. empty pot
   3. put leaves in pot
   4. pour water
   5. wait
   6. pour tea

Plan 0: do 1. 
        if pot is full, 
           then do 2 at the same time
        do 3-4-5
        when tea is brewed, do 6

Plan 1: do 1.1-1.2-1.3-1.4
        when water is boiling, do 1.5

Tea-making HTA model in textual notation

Digression - STRIPS

Control program for SHAKEY, developed at Stanford Research Institute

SHAKEY's Blockworld

SHAKEY's Blockworld

Early planner - hierarchical plans, constructed bottom-up

HTA constructs plans top-down, but decomposes tasks in a similar way

Cognitive Task Analysis

Knowledge based approach


Psychology of Human Computer Interaction, Card, Moran and Newell, 1983

Description of knowledge that a user must have in order to carry out tasks on a device or system
procedural knowledge

GOMS is based on the Model Human Processor:

Model Human Processor

Model Human Processor

GOMS is in fact a family of TA methods:

Keystroke Level Model - Card, Moran, Newell 1983
uses a single stream of operators
added goals, sub-goals and methods
Natural GOMS Language
based on Cognitive Complexity Theory
Cognitive, Perceptual, Motors
added parallel methods, subject to processing constraints. models use PERT charts

GOMS Explained

the state that the user wishes to achieve
action that the user executes
sequence of steps that accomplishes a goal
selection rule
used to route control to the approriate method to achieve a goal

GOMS Goals

things that a user must accomplish because of the design or nature of a device
characteristic of the general kind of task
the way that the user ``naturally'' characterises tasks

Natural goals may be related to device-independent goals.

GOMS Operators

observable user actions
internal user actions

Often defined by hardware.

GOMS Methods

Methods are used to accomplish goals and are composed of operators.

Mental operators allow sub-goals - hierarchical decomposition.

Describing methods is the focus of task analysis.

GOMS Selection Rules

Selection rules are needed if there is more than one method for a goal.

Common approach is to convert a general goal in a specific context into a specific goal.


Natural GOMS Language - Kieras, 1988

action-object pair
delete word
action-object pair
Click mouse button
specific to a goal, sequence of steps
Method for goal: foo
Step 1. ...
Step 2. ...
selection rule
list of alternatives with guards
Selection rule set for goal: foo
If cond Then accomplish goal: bar

David Kieras has written a guide to GOMS (and NGOMSL in particular) which is available at: <URL:ftp://ftp.eecs.umich.edu/people/kieras/Guide.ps>

NGOMSL Example

Method for goal: make tea
  Step 1. Accomplish goal: boil water
  Step 2. Decide: If Verify that pot is full
                  Then empty pot
  Step 3. put leaves in pot
  Step 4. pour water
  Step 5. Wait for tea to brew
  Step 6. pour tea

Selection rule set for goal: boil water
If kettle is electric 
Then Accomplish goal: boil-water-in-kettle
Else Accomplish goal: boil-water-on-stove
Return with goal accomplished

Method for goal: boil-water-in-kettle
  Step 1. fill kettle
  Step 2. turn kettle on
  Step 3. Wait for water to boil
  Step 4. Return with goal accomplished

Method for goal: boil-water-on-stove
  Step 1. fill kettle
  Step 2. light stove
  Step 3. put kettle on stove
  Step 4. Wait for water to boil
  Step 5. turn stove off
  Step 6. Return with goal accomplished

Tea-making NGOMSL Example

Digression - Production Rules

Common architecture in symbolic AI

Production Rule System

Production Rule System

Working memory contains facts. Production rules are matched against the contents of the WM. Those that match form the conflict set. The process of conflict resolution selects a rule from the conflict set which is then fired, adding new facts to the WM.

GOMS is partly based on earlier work with Production Rule Systems

The correspondence between PS concepts and GOMS concepts is straightforward:


Analyse the task of making a telephone call from a public telephone. Call payment may be in cash, or by credit card, phone card or reverse charges.

$Date: 1999-02-25 12:31:15+00 $