Fault Tolerance

The term "fault tolerance" originates in data processing, and describes the possibility of a system to continue working continuously even in the event of incorrect input or failure. With regard to usability, fault-tolerance means that a user can still achieve their goal by correcting incorrect inputs or functions with minimal effort. Fault-tolerance is also part of EN ISO 9241, which specifies criteria for the ergonomic design of user interfaces.


Software programs and websites are never free of errors even when optimally prepared and designed, and even after usability tests. Users can also make mistakes themselves through their own selection. To ensure that programs and websites continue to function and not crash due to user error, additional functions must be installed that offer a certain fault tolerance.

DIN ISO 9241

The fault tolerance of user interfaces is defined in the fifth paragraph of EN ISO 9241:

" A dialog is error-tolerant if the intended work result can be achieved with either no or minimal correction effort on the part of the user despite recognizable input errors. (Source: Ergo-online) "

Against this background, web forms, for example, should always be checked for plausibility or completeness of the entries. Similarly, error messages should not be formulated in technical terms, but in a generally understandable form, so that the user can trace the error and avoid it with little effort when trying again.

Elements of fault tolerance

Fault tolerance can be required for both hardware and software. With hardware, a parallel sequence is usually installed which can detect errors immediately. A third component then fixes it.

There are several areas of software that can be corrected by error tolerance:

  • Design errors
  • Error in data
  • Error in chronological sequence

With regard to usability, it is primarily the fault tolerance of user interfaces such as websites that is decisive. The following elements can increase the error tolerance:

  • Information on possible incorrect entries
  • Explanation of errors so that the user can correct them
  • Display where the error occurred
  • Automated correction with hints
  • Prompt to check user input

In a way, a 404 error page also contributes to fault tolerance. The server informs the user that the requested page is not available.