Hierarchical Task Analysis

Hierarchical task analysis (HTA) is a widely used type of Task analysis where a high-level task is decomposed into a hierarchy of subtasks. An HTA is sometimes referred to as a hierarchical decomposition.

Related Links

Annett, J., Duncan, K.: Task Analysis and Training Design. Occupational Psychology 41 (1967) 211-227

Peter Hornsby: Hierarchical Task Analysis. This article provides good insight on Hierarchical Task analysis and includes an easy to understand example. The article can be found at http://uxmatters.com/mt/archives/2010/02/hierarchical-task-analysis.php

Abe Crystal, Beth Ellington: Task analysis and human-computer interaction: approaches, techniques, and levels of analysis. This papers includes a description of the history of HTA, also identifying its strengths, weaknesses and approaches. In addition it compares HTA with other Task analysis techniques. The paper can be found at http://www.ils.unc.edu/~acrystal/AMCIS04-crystal-ellington-final.pdf

Formal Publications

• Annett, J. & Stanton, N. A. (Eds.). (2000). Task Analysis. London, UK: Taylor & Francis.
• Hackos, J. & Redish, J. (1998). User and Task Analysis for Interface Design. Chichester: Wiley.
• Kirwan, B. & Ainsworth, L.K. (Eds.). (1992). A Guide to Task Analysis. London, UK: Taylor & Francis.

Detailed Description

Benefits, Advantages and Disadvantages

Although HTA has been used for over 40 years, it is still widely used in industry because it is simple and straightforward. The results of an HTA is a starting point for more detailed modeling methods, like GOMS.

Advantages

• HTA is a simple and flexible method that does not depend on a methodological context.
• HTA enables the representation of a task hierarchy that could be further detailed.
• Although HTA is task oriented and to some extent user oriented it still maintains a strong relationship with traditional software engineering.
• HTA provides information, inefficiencies in tasks, that can be used for developing product requirements.

Disadvantages

• There are no strict rules for creating an HTA diagram so different analysts will generate inconsistent hierarchies at varying levels of detail.
• HTA requires both training and experience. It is not a tool that can be applied immediately.
• HTA is not a predictive tool. It focuses on existing tasks.
• HTA diagrams can become quite complex.

Cost-Effectiveness

When used in large project, HTA requires a lot of overhead work to revise / maintain task numbers and plans as tasks are edited and moved within the hierarchy. Also, it is difficult to synchronize the graphical and textual representations.

How To

Procedure

HTA representation consists of a hierarchy of tasks, sub-tasks and actions together with plans that describe the ordering and conditions of task performance. The plan is enabling the analyst to describe sequential, conditional and iterative tasks.

The decomposition in detail is done according to the P x C rule that estimates the efficiency of going on with the analysis based on the probability of inadequate performance and the cost of the inadequate performance. Therefore, further decomposition of tasks which requires extra effort of analysis is done only when there are reasons to estimate that current performance is not acceptable.

Typical reasons are error prone situation that could lead to serious consequences such as: inconsistent interaction methods, problems with learning the task delegation or execution time constraints. In all cases cost-benefit estimation is useful in order to avoid waste of time.

The method provides with both graphical and tabular representations.

Data Analysis and Reporting

Several tools have been developed to support HTA. See, for example TaskArchitect.