Reliability is “quality changing over time.” The everyday usage term “quality of a product” is loosely taken to mean its inherent degree of excellence. In industry, this is made more precise by defining quality to be “conformance to requirements at the start of use”. Assuming the product specifications adequately capture customer requirements, the quality level can now be precisely measured by the fraction of units shipped that meet specifications.
But how many of these units still meet specifications after a week of operation? Or after a month, or at the end of a one year warranty period? That is where “reliability” comes in. Quality is a snapshot at the start of life and reliability is a motion picture of the day-by-day operation. Time zero defects are manufacturing mistakes that escaped final test. The additional defects that appear over time are “reliability defects” or reliability fallout.
The quality level might be described by a single fraction defective. To describe reliability fallout a probability model that describes the fraction fallout over time is needed. This is known as the life distribution model.
Difference
The person who uses a product often uses the terms quality and reliability interchangeably, but they mean very different things.
Quality is the standard of something as measured against other things. It is the measure of excellence or state of being free from defects or deficiencies. From this definition, you assess the quality of something relative to something else, and what you measure is the result of its manufacture.
Reliability has two related definitions. One is the state of being dependable. The other is consistency – that is, the degree to which something yields the same or compatible result time after time. These definitions are related in that one is dependable based on his or her ability to do, act, say or behave in a consistent manner.
So where do quality and reliability come from? Reliability is a function of the design; quality is a result of the manufacturing. When a product is designed and handed over to manufacturing to produce, it is accompanied by a “specification.” The specification, or “spec,” describes the product completely and is the basis for determining quality, and becomes the standard against which it is measured.
If developing a new product is difficult, specifying it so manufacturing makes it as designed is just as difficult. When someone returns a product with a complaint and it is taken to manufacturing to inspect, manufacturing takes out the specifications they were given and compares the product to the specs. If the product conforms to the specifications, manufacturing will correctly say, “The quality is perfect.” So, the definition of quality is simply whether or not the product meets the specification.
Reliability is what engineers build into the product as part of the design and specification. The creative concept behind the design plays a huge role in determining the overall reliability of the design.
Reliability is built into a product as part of the design and selection of materials. In the end, it is a combination of great design, materials selection, specification and manufacturing that creates products that have both reliability and quality you can trust.
