reliability (www.merriam-webster.com) (dictionary) reliability = 1. the quality or state of being reliable

2. the extent to which an experiment, test, or measuring procedure yields the same results on repeated trials American society for quality (asq) defines reliability as “probability that a product, system or service will perform its intended function adequately for a specified period of time, operating in a defined operating environment without failure” components of this definition: intended function: for example, to light, cut, rotate or heat. satisfactory: perform according to a specification; with an acceptable degree of compliance. specific period of time: minutes, days, months or number of cycles. specified conditions: for example, temperature, speed or pressure. R. Dan Reid and Nada R. Sanders, authors of book operations management, define reliability as “the probability that a product, service, or part will perform as intended for a specified period of time under normal conditions”. as no of components of a product increases, the product's reliability reduces model a : the product has 2 components, each having reliability = 99% (0.99) therefore, reliability of model a of the product = 0.99 x 0.99 = 98% model b : the product has 5 components, each having reliability = 99% (0.99) therefore, reliability of model b of the product = 0.99 x 0.99 x 0.99 x 0.99 x 0.99 = 95% model c : the product has 10 components, each having reliability = 99% (0.99) therefore, reliability of model c of the product = 0.9910 = 90% increasing reliability of a product / service / system reliability of a product / service / system (M) can be increased by including backup components ie building in redundancy. redundancy costs money but is essential …

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a) if a failure of M will increase the probability of permanent physical impairment or even loss of a life example : electricity-supply system to an operation room of a hospital b) if a failure of M will cause deaths of few / many people examples : braking system of a car supply of heavy water to a nuclear reactor c) if cost of repair of M will be high example : airconditioning system of International Space Station (ISS) d) if there is no scope for a repair of M example : a satellite launched into outer space e) if a failure of M will compromise of security of nation concerned example : missile launch system of a warship f) if a failure of M will bring disrepute to nation concerned example : electricity-supply system to stadium where opening ceremony of an Olympic games edition will be held Redundancy is built into a product / service / system by placing components in parallel, so that when one component fails the other component takes over. In this case, … reliability of product / system = reliability of 1st component + (reliability of 2nd component x probability of needing 2nd component) the 2nd component is needed only when the 1st component fails. hence, probability of reliability of needing the 2nd component = probability of failure of 1st component = 1 – reliability of 1st component example no 1 a product has 2 components, the 2nd component being a backup for the 1st one.

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reliability of 1st component = 90% reliability of 2nd component = 90% what is reliability of the product ? reliability of the product = 0.9 + (0.9 x (1 - 0.9))

= 0.9 + (0.9 x 0.1) = 0.99 ie 99%

thus, reliability of the product has been increased from 90% to 99% by including a backup component example no 2 a product has 2 components, the 2nd component being a backup for the 1st one. reliability of 1st component = 80% reliability of 2nd component = 90% what is reliability of the product ? reliability of the product = 0.8 + (0.9 x (1 - 0.8))

= 0.8 + (0.9 x 0.2) = 0.98 ie 98%

thus, reliability of the product has been increased from 80% to 98% by including a backup component

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