We need to understand what a product should do when working to be able to detect when it has failed.
When a function does not perform as expected that is a failure. Being very clear about an item’s function(s) is vital when establishing reliability goals.
There are two elements to this concept.
First the list of functions a product should perform. This is often included in some detail within a product requirements document. The PRD includes primary and secondary functions, plus elements such as size, weight, color. The PRD may also include performance requirements such as frequency band and power for a radio, for example.
The second element of a function is the set of customer expectations concerning functions. This is often unstated, yet when a customer says a product has failed it may or may not be a PRD-related element that has failed. The customer probably does not have the PRD, yet does know when their expectation of a function is not satisfied.
Marketing, sales, and senior management often determine the set of functions for a product. They tend not to detail what is a failure, other than the implied definition of the lack of performance.
In some cases, this is not obvious, as the function may be degraded or partially functional. If a radio’s power output is insufficient for customer use, is that a failure? The device may be transmitting within specified values, yet if the customer expects a longer range, that is a failure from the customer’s point of view. Or, the transmitter may be faulty and not able to reach the intended range.
Each major element defines what is and what is not a failure. This may provide clarity to the design and testing teams to clearly understand if the product is meeting reliability requirements or not.