Task Analysis

Overview

Task Analysis
Hierarchical Task Analysis
Cognitive Task Analysis

Task Analysis

support user-centred design
help designs reflect user requirements

Uses of Task Analysis

documentation
requirements gathering
predict performance

Goals, tasks and actions

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

Hierarchical Task Analysis

Task decomposition approach

tasks
sub-tasks
actions
plans

HTA Structure Chart Notation

Structure Chart Notation

HTA Structure Charts

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

Early planner - hierarchical plans, constructed bottom-up

selection of primitive operators
- pickup(X)
- putdown(X)
- stack(X)
macro facility for combining operators
operator preconditions

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

Cognitive Task Analysis

Knowledge based approach

Goal, Operator, Method and Selection (GOMS)
Task Knowledge Structure (TKS)
Knowledge Analysis of Tasks (KAT)

GOMS

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

model: 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

GOMS is in fact a family of TA methods:

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

GOMS Explained

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

GOMS Goals

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

Natural goals may be related to device-independent goals.

GOMS Operators

external

observable user actions

press a key
wait for an event
move cursor

mental

internal user actions

make a basic decision
recall an item from working memory
set up a goal

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.

NGOMSL

Natural GOMS Language - Kieras, 1988

goal: action-object pair; delete word
operator: action-object pair; Click mouse button
method: 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

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:

production rules correspond to methods
conflict resolution corresponds to selection rules
contents of WM are goals
the means by which production rules assert goals are operators

Assignment

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 $