Post Completion Errors and How to Avoid Them

Picture this: It's your first day at your new job, and your boss asks you to photocopy a proposal for the next business meeting. Nervously, you glance at the photocopier and feel shocked and relieved to see that you can actually work out how to use it! However, in your haste to impress your boss and show him how efficient you are, you rush through the photocopying process and forget to take the original out of the photocopier. You proudly walk up to your boss, certain that the next task you will be assigned will be much more important and hand him the copies. He takes them from you and asks you for the original documents...and that's when it hits you! You forgot to take it out of the copier! You've just made a post completion error and doomed yourself to a lifetime of tedious tasks in the office.

Post completion errors are a specific type of human error that is completely preventable, in most cases, if the product has been designed properly. For example, a simple, well timed beep from the photocopier reminding you to take the original out of the machine (once it had printed out the last copy), would save you the hassle of having to remember to take the original out of the copier. Post completion errors are systematic in nature and only occur sometimes. They tend to occur when tasks follow a sequence of actions in order to achieve their goal and when users have to perform a sub-task after the main goal has been achieved (i.e. when the goal achieving step is not the final step in the sequence).

In the above example, no catastrophes occurred, however post completion errors can occur anywhere and cannot be trained out. They also tend to be hard to predict, a nuisance and can make the entire experience of using a product negative. Therefore it is important that designers and evaluators of products make an effort to eliminate the occurrence of post completion errors as early in the design and evaluation cycle as possible.

It is possible to attempt to eliminate post completion errors. One strategy is for designers and evaluators to watch out for tasks that consist of a sequence of sub-tasks and ensure that the goal achieving step is the final task in the sequence. For example, obtaining copies from a photocopier can be divided into the following sub-tasks: place original in the copier, provide details of number of copies, properties of copies etc, press copy button, take original out of the copier and collect copies. The goal achieving task in this example is the ‘collect copies' task. Therefore ensuring that this goal is achieved as the last task in the sequence eliminates the post completion error.

However certain situations demand that the task sequence cannot be altered. In these situations, ensuring that the user is given a well timed, relevant reminder or ensuring that the user's working memory is not over loaded can minimise the occurrences of the error.

As experts in human error and usability, we can help you to anticipate and design out or minimise post completion errors, ensuring that your users have a pleasurable experience using your product. Please contact us for more information.