General Philosophy of Prediction
Reliability can be defined in conceptual and quantitative terms:
• As a concept, reliability is the ability of an item to perform its specified function without failure under stated conditions for a stated period of time, number of cycles, distance or any other variate.
• As a quantitative measure, reliability is the probability that an item will perform its specified function for a specified interval under specified conditions.
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In the previous definitions, an item is any one of enumerated things, without regard to size or complexity. An item may therefore be a complete system at one extreme or a single component at the other.
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Four elements are involved either directly or indirectly in both of these definitions of reliability:
• Probability.
• Performance requirements.
• Time (or another variate).
• Conditions under which the item is used.
To predict reliability, therefore, relationships must be established between these four elements.