Hierarchical Task Analysis: Breaking Down Complex Tasks into Subtasks, Slides of Process Engineering

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Hierarchical Task analysis

Introduction

• What are Tasks

– What the user has to do (or thinks what he/she has to do) in

order to accomplish a goal

– Each task should be

• Meaningful
• Associated with a goal
• Identifiable by the user

• What is Task Analysis

– A process of analyzing the way people perform their tasks

• The things they do
• The things they act on
• The things they need to know

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Task Decomposition

• What is Task Decomposition
  • A top-down process in which a task is split into subtasks by sequence
• Aims
  • Describe the actions users do
  • Structure actions in a task-subtask hierarchy
  • Describe order of subtasks
• Hierarchical Task Analysis (HTA)
  • Outputs are a hierarchy of tasks and subtasks and plans describing in what order and under what conditions subtasks are performed
  • Shown as textual descriptions or diagrams
    • Information may be more accessible at a glance with diagrams, especially

in hierarchies with many levels

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1. Clean the house
   1. Get the vacuum cleaner out
   2. Get the appropriate attachment
   3. Clean the rooms 3.1. Clean the hall 3.2. Clean the living rooms 3.3. Clean the bedrooms
   4. Empty the dust bag
   5. Put vacuum cleaner and attachments away

Plans Plan 0: do 1 - 2 - 3 - 5 in that order. When the dust bag gets full do 4 Plan 3: do any of 3.1, 3.2 or 3.3 in any order depending on which rooms need cleaning

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Textual HTA of the Task of Cleaning a House

• Indentation is used to denote the levels in the task hierarchy
• Plans are labeled by the tasks they correspond to
• Only the plans denote the order of task performance
• Not all subtasks need to performed, and not necessarily in the order presented in the hierarchy

Generating Hierarchy

• Identify the Major Task to be Analyzed

– e.g. clean house, purchase a flight ticket online, copy a

ten-page paper, etc.

• Break Down the Major Task into Subtasks

– What subtasks must be accomplished in order to

perform the main task

– Refer to various sources (e.g. direct observation, expert

opinion, documentation, etc.)

– Try to be specific in terms of the objectives of subtasks

• Decide Upon the Level of Detail into Which to

Further Decompose the Subtasks

– Some stopping rule 7

Stopping Rule

• Depends on the Purpose of the Task Analysis

– Put more effort into those subtasks which are directly

relevant to the intended purpose

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0. In an emergency situation in a chemical plant

1. Read the alarms

2. Work out appropriate corrective action

3. Perform corrective action

• If our ultimate aim is to install computer monitoring of the plant, then

we would be interested in expanding subtasks 1 and 3

• If the aim is to produce online operations manuals, then subtask 2

would require expansion

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Make a cup of tea

Boil water

Empty pot

Put tea leaves in pot

Pour in boiling water

Wait for 4 or 5 minutes

Plan 0: do 1 At the same time, if the pot is full, do 2 Then do 3 – 4 After four or five minutes do 6

HTA of the Task of Making a Cup of Tea

Pour tea Plan 1: do 1.1 – 1.2 – 1. when kettle boils, do 1.

Fill kettle

Put kettle on stove

Wait for kettle to boil

Turn off gas

Any omission or error? Can some first-level subtasks be combined?

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Boil water

Empty pot

Make pot of tea

Pour in boiling water

Wait for 4 or 5 minutes

Fill kettle

Put kettle on stove

Wait for kettle to boil

Pour tea

Plan 1: do 1.1 – 1.2 – 1.3 – 1. when kettle boils, do 1.

1.5. Turn off gas

Warm pot

Put tea leaves in pot

Plan 3: do 3.1 – 3.2 – 3.

Turn on gas

Can we expand 5?

make a cup of tea Plan 0: do 1 At the same time, if the pot is full, do 2 Then do 3 – 4 After four or five minutes do 5

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make cups of tea

Boil water

Empty pot

Make pot

Pour in boiling water

Wait for 4 or 5 minutes

Plan 0: do 1 At the same time, if the pot is full, do 2 Then do 3 – 4 After 4 or 5 minutes do 6

Fill kettle

Put kettle on stove

Wait for kettle to boil

Pour tea

Plan 1: do 1.1 – 1.2 – 1.3 – 1. when kettle boils, do 1.

1.5. Turn off gas

Warm pot

Put tea leaves in pot

Plan 3: do 3.1 – 3.2 – 3.

Turn on gas

Put milk in cup

Fill up with tea

Add sugar

Plan 5:

5.1 5.2 (^) More cup(s)? Yes

Handling Waiting

• Considered as a subtask if

it is a “busy” waiting

– e.g. The person may be

chatting while the tea

brews

• Included in the plan if

time seems critical for the

task sequence

• Perhaps a little redundant

in this example, but task 14

Redundant? Plan or subtask?

Types of Plan

• Cycles

– Repeat some subtasks until a condition is reached

• e.g. Repeatedly perform subtasks 5.1 – 5.3 until no more cup is left in the HTA of tea making

• Time Sharing

– Some subtasks can be done at the same time

• e.g. Subtasks 1 and 2 can be done at the same time in the HTA of tea making

• Discretionary Subtasks

– Whether to perform some subtasks is at the people’s

discretion

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