The formal definition of reliability is …
“The probability that an item will perform its intended function under stated conditions, for either a specified interval or over its useful life.”
The three key terms in this statement are
- Intended Function
- Stated Conditions
- Specific Interval
A reliability statement made without all three of these factors has no meaning. Communicating that a product has a reliability of 99.999% means nothing. 99.999% reliability in what capacity? “What if I use it as a boat anchor? Does it have a 99.999% reliability in that function?” Will it have that 99.999% reliability in 25 years?” “Is it ok if I leave it outside when I’m not using it?”
A complete statement of reliability for a hard drive may be
- “99.999% reliability when used for data storage, interfacing with a Mac OS in an indoor environment class A, over a 5 year period.”
- “99.95% reliability when used for data storage, interfacing with a Mac OS in an indoor environment class B/C, over a 5 year period.”
- “99.2% reliability when used for data storage, interfacing with a Mac OS or Windows OS in an indoor environment class B/C, over a 10 year period.”
It’s difficult early in a program to be diligent and ensure all these factors are defined and captured. Having experienced the unnecessary iterations that occur in test and design due to fluctuating or ambivalent definitions; I can vouch for the value of getting these parameters and definitions complete as early as possible.
-Adam
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