Reliability Learning 002

Quality versus reliability
Reliability is
The everyday usage term "quality of a product" is loosely taken to
"quality
mean its inherent degree of excellence. In industry, this is made more
changing
precise by defining quality to be "conformance to requirements at the
over time"
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.
A motion
But how many of these units still meet specifications after a week of
picture
operation? Or after a month, or at the end of a one year warranty
instead of a
period? That is where "reliability" comes in. Quality is a snapshot at the
snapshot
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.
Life
The quality level might be described by a single fraction defective. To
distributions
describe reliability fallout a probability model that describes the
model
fraction fallout over time is needed. This is known as the life
fraction
distribution model.
fallout over
time