Road Network Operations
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Human Tasks and Errors

Analysis of tasks and errors is a hugely important activity for anyone seeking to understand how users interact with ITS or wishing to create the environment in which the interaction will take place – between users or between a user and an object/activity. Task analysis requires understanding and documenting all aspects of an activity in order to create or understand processes that are effective, practical and useful. Typically a task analysis is used to identify gaps in the knowledge or understanding of a process. Alternatively it may be used to highlight inefficiencies or safety-critical elements. In both cases the task analysis provides a tool with which to perform a secondary function – usually design-related. Error analysis is a specific extension of a task analysis and is about probing the activities identified by the task analysis – to determine how and why a user might make an error, so that the potential error can be designed out or the consequences can be mitigated.

Task analysis

The person conducting a task analysis first has to identify the overall task or activity to be analysed, and then to define the scope of the analysis. For example, it may be that they wish to examine the tasks performed by an operator at a monitoring station, but are only interested in what the operator does when they are seated at an active station. This would be the defined scope of the analysis.

Within this overall activity all the key subtasks that make up the overall activity must be identified. It is up to the person undertaking the analysis to decide what represents a useful and meaningful division of subtasks. This is something that comes partly from experience of performing such analyses and partly from understanding of the activity being analysed. Crucially, each subtask should have a definable start and end point.

With the subtasks created, the investigator then defines a series of rules and conditions which govern how each subtask is performed. For example, it may be that the monitoring station operator has at their workstation a series of monitoring systems (subtasks) where each subtask is distinct and separate – and it may be that the operator must perform the subtasks in a pre-defined sequence. The investigator must specify the rules governing how each subtask relates to the others. For example: “perform subtasks A and B alternately. At any time, perform subtask C – as and when required”.

The investigator would then look at each subtask in turn and perform a similar process to the process described above. The subtasks that make up subtask A would be identified and the rules governing their commission defined. This process of hierarchical subdivision continues until either there is no more meaningful division of tasks that can be performed – or the investigator has reached a level of understanding useful enough to inform the design.

Purpose of analysis

Performing a task analysis allows a researcher to identify the following:

  • areas where they have a gap in understanding or knowledge
  • subtasks that do not have clearly defined procedures
  • ambiguities in the division of labour and responsibilities
  • safety critical processes or those integral to system operation
  • inefficiencies at subtask or overall task level

An error analysis builds on the task analysis and requires a similar approach. It looks at all the different ways in which an operator or outside agent could perform an error in each subtask identified. For example, one subtask for the operator of a monitoring station, may be to activate an alert system. Errors could include (among others), selecting the wrong incident response plan, pressing the wrong button, failing to push a lever all the way, looking at the wrong screen or dial, or activating the system at the wrong time. Typically such an analysis would be performed by considering each of a list of possible error mechanisms in turn, to avoid missing potential errors. Again, a combination of experience in conducting error analysis and an understanding of the workings of the overall activity are useful.

Advice for Practitioners

Before conducting a task or error analysis it is important to define what the output of the analysis is to be used for, as this will influence how the analysis is performed. For example, it may be that the investigator is only be interested in particular subtasks and activities – or that a particular level of detail is required, below which the analysis is useless and beyond which it is simply a waste of time and effort. Knowing the level of detail required is a key parameter as without this cut-off point, the analysis could go on almost indefinitely. A basic task analysis is a useful way for anyone to gain a clearer picture of any working environment. For more detailed analyses or situations where the task analysis is to provide the foundation for a larger set of activities, it is advisable to acquire the services of experienced practitioners.

task analysis

The following is a basic overview of the key principles/stages:

  • establish what it is you wish to know and how you intend to use the information
  • identify and clearly define the scope of the task/environment to be analysed

break the overall task down into stages:

  • identify key subtasks within the parent activity
  • establish the rules that govern how and when each subtask starts and finishes
  • repeat the previous step for each of the subtasks identified
  • continue this process until a sufficient level of detail has been reached (specifically, when the information is available to answer the original question)

complimentary error analysis

A task analysis is often useful in its own right. It may also be useful to conduct a complimentary error analysis. Again it is best to use an experienced professional for large-scale analyses – but for a basic assessment, the following method can be applied:

  • identify the complexity required and decide what level of subtask breakdown is required
  • examine each subtask to determine all the possible ways in which a user might perform an error in carrying out that subtask (this requires the practitioner to understand the task being analysed)
  • consider what external factors might make such errors more likely or more severe (performance-shaping factors)
  • determine what precursor events, actions or omissions are required to happen in order to make the error possible or the performance shaping factors more relevant
  • for each error, identify ways to reduce the potential for error or to mitigate its consequences – bearing in mind that it is preferable to prevent an error from occurring than to try to mitigate the consequences


Reference sources

Wilson, J. and Corlett, N. (2005). Evaluation of Human Work (third edition). Taylor and Francis.