Task analysis: hierarchical task analysis and GOMS

Task analysis writes down how a goal is reached

Intuition

Before designing a flow for making tea, you list what the person does and what they think about while doing it. Task analysis comes in two flavours: the steps, and the thinking behind the steps.

Lecture 5 gives two approaches. Logic of task analysis records the sequence of steps required to achieve a goal, in text or as a diagram. Hierarchical task analysis (HTA) is the example. Cognitive task analysis records the cognitive processes required to achieve the goal. Cognitive walkthrough and GOMS are the examples.

HTA decomposes a goal into subtasks with plans

Task decomposition is a list of tasks with indentation to denote decomposition. The lecture example is house cleaning.

Mechanism

0. In order to clean the house. Plan 0: do 1, 2, 3 and 5 in that order, and when the dust bag gets full do 4.
1. Get the vacuum cleaner out.
2. Get the appropriate attachment.
3. Clean the rooms. Plan 3: do any of 3.1, 3.2 or 3.3 in any order depending on which rooms need cleaning.
   • 3.1 Clean the hall. 3.2 Clean the living rooms. 3.3 Clean the bedrooms.
4. Empty the dust bag.
5. Put the vacuum cleaner and attachments away.

HTA is a graphical representation of task structure using structure chart notation. Sequence is usually shown left to right and annotations indicate plans. Steps: identify the task, break it into subtasks (recommended 4 to 8), decide the level of detail and decompose further, determine the plans (they dictate how tasks and subtasks are achieved), then draw the tree with the goal at the top, subtasks in the next layer left to right, and decompositions below.

Exam detail

Outcomes of HTA: manuals and tuition (a procedural how-to manual and a conceptual manual) and detailed interface design (task frequency guides default choices, existing task sequences guide dialogue design). The plan source on the slide is Stanton et al. (2005), Human factors methods.

Pitfall

Plans are not extra tasks. They describe the order or conditions under which subtasks run, such as “any order” or “do 4 when the bag is full”. The tree itself holds the tasks.

GOMS models the user’s goals, actions and choices

Mechanism

• Goals: what the user wants to achieve.
• Operators: the basic actions the user must perform to use the system.
• Methods: sequences of subtasks and operators.
• Selection rules: the rules people use to choose between methods of achieving the same subtask.

Try it: worked example

Use GOMS to describe saving a document with the menu or with Ctrl+S.

Show answer and steps

AnswerGoal: save. Methods: menu route and shortcut route. Selection rule: use the shortcut when hands are on the keyboard.

1. Goal: save the document.
2. Operators: move the pointer, click, press keys.
3. Method 1 (menu): click File, then Save, so the operators are move and click twice.
4. Method 2 (shortcut): press Control and S together.
5. Selection rule: choose the shortcut when your hands are already on the keyboard, otherwise use the menu. The rule is an illustration, since the lecture slide only defines the four elements.

Recall

What do the letters GOMS stand for?

Goals, Operators, Methods, Selection rules.