A short essay on establishing meaningful reliability goals for a project or product.

1. f m sre liabilit y.co m
http://www.fmsreliability.co m/educatio n/reliability-go als/
Reliability Goals
RELIABILITY GOALS
T he target, objective, mission or goal is the statement that provides a design team with f ocus and direction. A
well stated goal will establish the business connection to the technical decisions, related to product durability
expectations. A well stated goal provides clarity across the organization and permits a common language f or
discussing design, supply chain and manuf acturing decisions.
Let’s explore the def inition of a ‘well stated reliability goal’. First, is it not simple MT BF, “as good as or better
than…” or ‘a 5 year product’. T hese are common ‘goals’ f ound across many industries, yet none permit a clear
technical understanding of the durability expectations f or the product.
T he common def inition f or reliability is
Reliability is … the ability or capability of the product to perf orm the specif ied f unction in the designated
environment f or a minimum length of time or minimum number of cycles or events.
(Ireson, Coombs et al. 1995)
Note this def inition has f our elements:
• Function
• Environment
• Duration
• Probability
Function

2. T he f unction is what the product is to do or perf orm. For example, an emergency room ventilator is to provide
assisted breathing f or a person. T his requires the ventilator to produce breathable air within a range of
pressures, within a prescribed cycle of respiration. It may include requirements f or f iltering, temperature, and
adjustments to pressure and timing of the cycle, etc. Of ten, a product development team either develops or is
given a detailed set of f unctional requirements.
T he f unctional elements of a product are of ten directly measurable. Further, the quality f unction of most
organizations verif ies that the design and production units meet the f unctional requirements. When the
product does not met the f unctional requirements it is considered a product f ailure. Within the f unction
def inition are the most important f unctions, which must not f ail, as well as the f unctions that upon f ailure may
simply degrade perf ormance, if noticed by the customer at all.
Environment
T he environment could be considered the weather around the product when in use. ‘Weather’ includes such
things as temperature, humidity, UV radiation intensity, etc. It should also include environmental f actors that
provide destructive stresses, such as vibration, moisture, corrosive gases, voltage transients, and more.
Another element of the environment is the use of the product. What is the use prof ile? Maybe once a day f or a
f ew minutes, like a remote control f or the stereo system. Or perhaps its a 24/7 operation, such as f or a server
system processing transactions f or a major online store. T he prof ile may include details concerning human
interactions, operating modes, shipping, storage, and installation. T he environmental conditions need to detail
how the product responds or degrades to the set of stresses the product encounters. T he environmental
conditions f ocus on the drivers f or the product’s most likely f ailure mechanisms.
Duration
T he duration is the amount of time or number of cycles the product is expected to f unction. A computer printer
may be expected to print f or f ive years. A washing machine is expected to wash clothes f or 10 years. An
implanted hearing aid is expected to last the lif e of the patient; if the patient is a child this expectation may be
more than 70 years.
T he duration expectations may be def ined by contract, market expectations, or by a business decision. T he
duration or lif e expectancy most likely is not the warranty period. For example, many personal computers have
a 3 month or 1 year warranty period. Yet, the product is expected to last at least two years or more with normal
use.
Many products have multiple durations that are of interest.
• Out of box
• Warranty
• Design Lif e
T he initial, out-of -box, or installation period is that duration when the customer is f irst setting up and using
the product. Brand visibility is at its highest and the expectation that a new product will f unction as expected is
very high. T he types of f ailures that may occur include installation or conf iguration errors, mistaken purchase,
shipping or installation damage, or simply buyer error. All of these ‘f ailures’ cost the company producing the
product resources.
T he warranty period is the duration associated with the producer’s promise to provide a product f ree of
def ects f or a stated period of time. For example a computer may have a 1-year warranty period. During this
one year, if the product f ails (usually limited to normal use and operating environment) the producer will repair
or replace the product. Naturally this will cost the producer resources.

3. T he design lif e is the business or market expected product duration of f unction use. Af ter the warranty period
there isn’t an expectation f or the producer to replace or repair the product, yet the customer may have a
reasonable expectation that the product will f unction satisf actorily over the design lif e duration. For example,
many cell phones have a 3-month warranty, yet as consumers we have an expectation that the phone will
f unction f or two years or more.
Marketing or senior management may set the design lif e. T hey may want to establish a market position f or the
product related to reliability. One way is to design a very robust product with a long design lif e duration. HP
calculators of ten have only a 3-month or 1-year warranty, yet many have lasted 10 or more years. T hese
calculators are known f or their robustness and of ten cost more to purchase – a reliability premium.
Each of the three durations of ten involves dif f erent risks related to the f ailure mechanisms. It is rare f or
bearings to wear out in the f irst 30 days, yet more likely f or a 10-year design lif e. Establishing three or more
durations within the product reliability goal permits the design team to f ocus on and address the f ull range of
product reliability risks.
Probability
T he probability is the likelihood of the product to survive over a specif ied period of time. In the f ormal
def inition of reliability above, the phrase ‘ability or capability’ ref ers to the probability. T his is the statistical part
of the reliability goal and without it the goal is f airly meaningless. Furthermore, stating a probability without an
associated duration and distribution is also meaningless in most cases.
What is the chance that a particular product will f unction as expected over the entire expected design lif e? How
many of the installed units will be f unctional over the warranty period? Since each product and the associated
environmental stresses vary, the use of statistics is unavoidable in describing product reliability. Even the
def inition of a product f ailure may vary by customer.
While there are many common terms to convey the probability of survival, the use of a percentage surviving is
the easiest understood and most easily applied across an organization. Stating that 95% of units are expected
to survive over the 5-year design lif e, means 95 out of 100 units will f unction properly over that 5-year period.
A similar statement is that not more than 5% of products f ail over the f ull f ive years. Or, may be stated as not
more than a 1% f ailure rate per year.
A common probability statement is the inverse of the f ailure rate, or MT BF. T he 95% reliability over 5 years (t)
becomes approximately 100 years MT BF (θ). T his does not mean the product will last 100 years, it does mean
that 95% of the products are expected to last 5 years.
Finally, stating a separate f ailure probability f or each duration of interest provides a set of
duration/probability couplets that permit dif f erent f ocus f or early or out of box f ailure risks, versus the longer
term f ailure risks.

4. Sometimes the product has a specif ic mission time, like an aircraf t with an expected 12-hour mission over a
20-year serviceable lif e period. T he probability of success f or the 12-hour mission time is maybe set relatively
high. It may also have a conditional probability, considering the number of missions since the last major service.
Some products have availability goals and undergo routine maintenance or repair. T hese products and many
complex systems require additional complexity in their goal setting. For the purpose of this discussion, we are
considering simple products that are not normally repaired, or products where the main interest is in the time to
the f irst f ailure.
T he point is that setting the reliability goal f or a product is not as simple as stating a ‘f ive year lif e’ – it requires
a clear statement with suf f icient detail of each of the f our elements: f unction, environment, duration, and
probability. It should also typically include at least three duration/probability couplets. T he goal establishes the
direction or target f or the entire design, supply chain and manuf acturing team.