Goals Operators Methods Selection Rules Computer Science Essay


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Is a more physiological approach to task analysis. As well as concentrating on the physical interactions between the user and interactive system, the mental aspect of the user is taken into account.

12.1 Task Description & Analysis

Task description and analysis are tools and techniques used by designers to identify and understand the interactions that take place between the user and the interactive product. Task description, comprising of scenarios and use cases are previously discussed in depth in lecture 10. This lecture examines the different task analysis tools that are used by designers for developing interactive products.

In human computer interaction, task analysis is a list of actions or tasks a user carries out while using an interactive system. Task analysis is used by designers as a way of identifying all the interactions that occur between the user and the interactive product, regardless of its size and significance. This allows designers to gain a more informative understanding of the various ways an interaction can take place as well as the various durations of these interactions. Different users' interactions are analyzed and the duration of time taken to complete an interaction is recorded. As well as this, the user's expertises are taken into account. For example if the user is a beginner in using the interactive product he may take longer than an expert user. This said, sometimes the user's expertise is not a fair reflection of the usability of the interactive product due to the interface of the product being an un-friendly one. Task analysis is a major tool used by designers to analyze, evaluate and improve system interfaces. Improving interfaces and interactive products as a whole is achieved by the ability to identify problems which task analysis facilitates. As well as finding problems in an interactive product, task analysis allows designers to collect and organize information about these problems and as a result, resolve them. A number of task analysis techniques are used by designers, including;

Hierarchical Task Analysis (HTA)



Goals-Operators-Methods-Selection rules (GOMS)

Key-Stroke-Level (KLM)

12.1.1 Hierarchical Task Analysis (HTA)

HTA is a technique that was created by John Annett and Keith Duncan in 1967. It is best described as a "systematic method of describing how work is organized in order to meet the overall objective of the job". The main goal of an interaction is identified and the different tasks that need to be carried out by the user to achieve this goal are divided up in a top-down fashion starting from the initial tasks and moving onto sub-tasks that follow and so on. In the HTA context, the various tasks carried out by the user are called operations and the different conditions that determine what operations are carried out are called plans.

HTA is initiated by stating the overall goal/objective of the interaction between the user and the interactive product. This initial objective is then sub-divided into operations and accompanied by a plan determining when these operations can be carried out by the user. These sub-operations are then further reduced into sub-operations with a plan. Plans are a key important of HTA as they tell the designer what information is needed from various operations before a separate operation can be carried out. HTAs can be illustrated in two ways; written form (see Listing 12.1) and using a diagram (see Figure 12.1). The written format does not take into account the conditions (plans) that are put in place in order for operations to occur, unlike the diagrammatic HTA where plans are used (highlighted by red color in figure 12.1). HTA is mostly used for projects of considerable size as there will be huge number of possible operations involved in an interactive system. This can become very confusing and difficult to follow as in the case of the diagrammatic HTA, the diagram can be enormous in size. As for the written HTA, it lacks the conditions that are needed for the various operations. On the other hand, HTA can be a very useful technique when used for focusing on a specific set of operations and objective, not taking into account all the possible operations and objectives of the whole interactive system. This focus allows designers to divide the various operations into manageable ones where information gathering is carried out, in regards to analyzing, evaluating and improving operations for the end user.

Heat up a pre-cooked meal using the microwave

Take meal out of freezer

Un-package meal

2.1. Place meal inside microwave

2.2. Choose the de-frost option

2.3. Choose the duration for de-frosting

2.4. Choose the heating option

2.5. Choose the duration for heating

3. Take out heated meal

4. Eat and enjoy

Listing 12.1: Written HTA

Heat up pre-cooked meal using microwave


Take meal out of freezer


Un-package meal


Take out heated meal


Eat and enjoy


Place meal inside microwave


Choose the de-frost option


Choose the duration of de-frosting


Choose the heating option


Choose the duration of heating


Plan 0,

Do 1, if 1 is done then do 2 and once 2 is done then do 3 and then 4

Plan 2,

Do 2.1, 2.2, 2.3, 2.4 and 2.5

Figure 12.1: Diagrammatic HTA

12.1.2 Goals-Operators-Methods-Selection rules (GOMS)

The second task analysis technique is the GOMS technique, developed by Stuart Card, Thomas P. Moran and Allen Newell in 1983 as part of their book, "The Psychology of Human Computer Interaction". This technique is designed to identify and measure the complexity of an interactive product. Similar to the HTA, GOMS makes use of methods to reach a user's goals. A method is made up of a series of operators that a user undertakes to achieve a goal. A selection rule is used if there is more than one method that can be used to achieve a goal. The GOMS technique is widely used to obtain an idea of the duration of the various interactions that can take place between a user and an interactive product. Each goal is accompanied with various operations that need to be done by the user for it to be achieved. In addition to this, each operation has a predicted time. The usage of time prediction allows the designers of the interactive product to gain a clearer picture of the different parts of the product and identify which parts will prove to be more difficult to interact with compared with other parts. However, time predictions are based on the assumption that an expert user is interacting with the system and no mistakes are made. As well as this, due to GOMS being a technique based on the interaction of an expert user with the interactive product, GOMS is also used to produce "help systems" that accompany the actual interactive product. This acts as guide for users, explaining how certain interactions can be performed.


Goals are the destination a user wants to reach while using an interactive product. They are the objectives and aims of the user. Depending on the amount of operations and complexity of the interaction, goals can also be divided into sub-goals. Using the example from the HTA technique, the goal of the user is to heat up a pre-cooked meal using a microwave.


Operators are the basic interactions that need to be carried out by the user to achieve the goals. These can either be physical interactions or psychological interactions between the user and interactive product. Unlike goals, operators cannot be broken down into sub-operators and each operation has an assigned amount of time for execution. Placing the pre-cooked meal into the microwave can be considered as an operation, taking 5 seconds to accomplish.


Methods are similar to operators in the sense that they are also interactions that are carried out by the user to reach the goal. However, methods can be described as a combination of operations and sub-goals which once they are completed, the overall goal is achieved. Taking the pre-cooked meal out of the freezer is a method, taking 0.5 seconds to accomplish.

Selection Rules

Selection rules, similar to conditions or plans are a set of rules that determine which method is used to accomplish a certain goal. Due to certain methods being only doable once another method is achieved, selection rules are there to make sure the correct order of methods in terms of their execution is realized. Selection rules can be considered as 'IF' statements that are placed on the various methods. Un-packaging the pre-cooked meal can only be carried out IF the meal has been taken out of the freezer.

A simplified version of the GOMS technique is the Key-Stroke-Level technique. Developed by Stuart Card and Thomas P. Moran, it concentrates on the simplified operators that take place during an interaction and does not accommodate any conditions or selection rules. Each operation is assigned a predicted time to execute. These operators are classified into six categories;

K - Pressing a key

P - Pointing to a location on the interface using a mouse

H - User physically moving his hands, either to use the mouse or keyboard

M - The user mentally preparing to carry out an operation

R - The time the user has to wait for the interactive product to response

The usage of the above categories is illustrated by the following example;

For his school project, Mustapha is going to write a report about the solar system. To write this report he is using Microsoft Word and is going to have a size 24 title ("Mustapha's Solar System Project) across the top of the front page. This title will be red in color. He is also going to write his name under the title in italics, size 14.

The Key-Stroke-Level technique illustrates the above example as follows;



Time (seconds)

Reach for mouse

H (mouse)


Move cursor to Microsoft Word icon

P (menu icon)


Click on icon to open

K (mouse)


Move cursor to specify the font size and color

P (menu icon)


Click on first page to specify where title will be

K (mouse)


Move hands in a position to start typing

H (keyboard)


Type the title of the project

M (title of project)


Move hand and click on the enter button to start a new line

H (keyboard)


Type his name

M (title of project)


Move cursor to the save icon

P (menu icon)





In this lesson, the following has been discussed:

In this lesson, both task description and analysis are discussed, with a focus on task analysis as two techniques are identified; Hierarchal Task Analysis (HTA) and Goals-Operators-Methods-Selection rules (GOMS). Their strengths and weaknesses are explored. As well as this, the various interaction methods in regards to interacting with an interactive product are identified.


The Waterfall lifecycle model is the most traditional and oldest model used by designers. (True)

The Waterfall model is considered as complex. (False)

The Spiral lifecycle model focuses on risk analysis and management. (True)

The Star lifecycle model focuses on the process of evaluation. (True)

HTA is used for projects that are considerably big in size and complexity. (True)

GOMS technique uses 'operations' and 'plans'. (False)

One of GOMS' outcomes is the production of a 'help document' which acts as a help guide for users. (True)

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