Task Analysis

Identifying needs and establishing requirements

Task analysis

• Task descriptions are often used to envision new systems or devices • Task analysis is used mainly to investigate an existing situation • It is important not to focus on superficial activities What are people trying to achieve? Why are they trying to achieve it?

How are they going about it?

• Many techniques, the most popular is Hierarchical Task Analysis (HTA)

Hierarchical Task Analysis

• Involves breaking a task down into subtasks, then sub-sub-tasks and so on. These are grouped as plans which specify how the tasks might be performed in practice • HTA focuses on physical and observable actions, and includes looking at actions not related to software or an interaction device • Start with a user goal which is examined and the main tasks for achieving it are identified • Tasks are sub-divided into sub-tasks

Hierarchical Task Analysis

0.

In order to borrow a book from the library 1.

go to the library 2.

3.

4.

find the required book 2.1 access library catalogue 2.2 access the search screen 2.3 enter search criteria 2.4 identify required book 2.5 note location go to correct shelf and retrieve book take book to checkout counter

Hierarchical Task Analysis (plans)

plan 0: do 1-3-4. If book isn’t on the shelf expected, do 2-3-4.

plan 2: do 2.1-2.4-2.5. If book not identified do 2.2-2.3-2.4.

Hierarchical Task Analysis (graphical

) Borrow a book from the library 0 plan 0: do 1-3-4. If book isn’t on the shelf expected, do 2-3-4.

go to the library 1 access catalog 2.1

find required book 2 retrieve book from shelf 3 take book to counter 4 plan 2: do 2.1-2.4-2.5.

If book not identified from information available, do 2.2-2.3-2.4-2.5

access search screen 2.2

enter search criteria 2.3

identify required book 2.4

note location 2.5

Predictive models

• Provide a way of evaluating products or designs without directly involving users • Psychological models of users are used to test designs • Less expensive than user testing • Usefulness limited to systems with predictable tasks - e.g., telephone answering systems, mobiles, etc.

• Based on expert behavior

GOMS

(Card et al., 1983) • Goals - the state the user wants to achieve e.g., find a website • Operators - the cognitive processes & physical actions performed to attain those goals, e.g., decide which search engine to use • Methods - the procedures for accomplishing the goals, e.g., drag mouse over field, type in keywords, press the go button • Selection rules - determine which method to select when there is more than one available

Keystroke level model

GOMS has also been developed further into a quantitative model - the keystroke level model.

This model allows predictions to be made about how long it takes an expert user to perform a task.

Response times for keystroke level operators

(Card et al., 1983) Operator K P P1 H M R(t) Description Pressing a single key or button Average skilled typist (55 wpm) Average non-skilled typist (40 wpm) Pressing shift or control key Typist unfamiliar with the keyboard Pointing with a mouse or other device on a display to select an object.

This value is derived from Fitts’ Law which is discussed below.

Clicking the mouse or similar device Bring ‘home’ hands on the keyboard or other device Mentally prepare/respond The response time is counted only if it causes the user to wait.

t Time (sec) 0.22

0.28

0.08

1.20

0.40

0.20

0.40

1.35

Summary

• Task analysis techniques such as HTA help to investigate existing systems and practices • Predictive models are used to evaluate systems with predictable tasks such as telephones.

• GOMS, Keystroke Level Model, & Fitts’ Law predict expert, error-free performance.