The reliability goal is a key element across the entire product lifecycle. From product definition to determining warranty to judging performance, knowing the goal in clear terms sets the stage for a successful product.
Reliability in engineering terms is the probability of satisfactory product performance within a defined environment over a stated duration.
The product requirements document (PRD or similar document) provides the list of product functions, often including size, weight, interface, and related product definition. The environment may or may not be defined with the PRD and may only be paraphrased, i.e. ground benign, North America Hospital, commercial aircraft cabin. Organizations often have clear product definition and at least a pretty good idea of the environment. Some even understand how often the product will be in use.
More common is the lack of or rather unclear statement of product reliability regarding probability of success and duration.
Stating MTBF is not sufficient.
O’Connor and Kleyner [page 1 of Practical Reliability Engineering, 5th Edition] define reliability as
The probability that an item will perform a required function without failure under stated conditions for a stated period of time.
Let’s look at each the four elements in turn. Probability – scary statistics included, is the chance that a unit will survive over a period of use, or the proportion of units that would be expected to survive a specific period of time. This is being positive minded. Likewise for my pessimist friends, one minus the probability of success if the probability of failure.
Duration – or, as stated the period of time. This can be hours of operation, calendar months, the number of printed pages, odometer miles, etc. Duration is some meaningful representation related to the irreversible passage of time. It is not MTBF, even though it is stated often in terms of hours, MTBF is the inverse of a failure rate, not time.
Function – easy until you look at it closely. You can return a product for a myriad of reasons. Wrong size, color or texture, doesn’t work, broken, or even made a mistake when buying the item. In all cases, the product did not do what the customer expected. Are all of these a failure? Yes, and no. Yes, in that some part of the product, it’s marketing or sales literature lead the customer to determine the product did meet their needs or provide the expected solution. We customarily separate the failures in expectation or purchase mistakes as not a failure of the product. We focus on what the product was designed to accomplish. Just be clear about what is meant for a product to ‘function’ and equally what is meant by ‘failure.’
The environment is the weather and use conditions. Weather, like temperature, humidity, hail, lightening strikes, dust, insects, chemical exposure, and similar elements that come into contact with the product. Some environments are carefully controlled, like a computer room, other are not, like a transformer on a telephone pole. For a complete goal statement, knowing if the product is expected to protected from rain or not may be important, for example. Setting clear expectations and fully characterizing the weather is critical.
In addition to weather, the environment may include the use profile. For example, an aircraft is exposed to one set of conditions on the ground, another during takeoff, and another during high altitude cruising. And, how often does it make the transition from one use mode to another? For the environment think about where and how often the product is used.
That is a basic overview of the four elements within a complete reliability goal statement. I highly recommend that these should be clearly stated and all in one place, not scattered across the product data sheet or requirements document.
Reliability Function (article)
3 Elements of Reliability Goal Setting (article)