Reliability theory is a body of ideas, mathematical models, and methods directed to predict, estimate, understand, and optimize the lifespan and failure distributions of systems and their components (adapted from Barlow and Proschan, 1975). Reliability theory allows researchers to predict the age-related failure kinetics for a system of given architecture (reliability structure) and given reliability of its components.
This introductory-overview chapter sets a stage for introducing theoretical models of systems failure in DEFINITIONS OF SYSTEM'S FAILURE AND AGING
aging. Mathematical models of systems failure are The concept of failure is important to the analysis of important for the studying of human aging, because system's reliability. In reliability theory failure is defined aging is associated with increased risk of failure in human as the event when a required function is terminated physiological systems. Theoretical analysis of systems (Rausand and Houland, 2003). In other words, failure failure in aging invites us to consider the general theory is such an outcome when the system deviates from
Handbook of Models for Human Aging
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optimistically anticipated and desired behavior ("fails"). Failures are often classified into two groups:
1. degradation failures, where the system or component no longer functions properly, and
2. catastrophic or fatal failures—the end of system's or component's life.
Good examples of degradation failures in humans would be an onset of different types of health impairments, diseases, or disabilities, while catastrophic or fatal failures obviously correspond to death. The notions of aging and failure are related to each other in the following way: when the risk of failure outcomes increases with age (''old is not as good as new'')—this is aging by definition. Note that according to reliability theory, aging is not just growing old; instead aging is a degradation leading to failure (adverse health outcomes)—becoming sick, disabled, frail and dead. Therefore, from a reliability-theory perspective the notion of ''healthy aging'' is an oxymoron like a healthy dying, or a healthy disease. More appropriate terms instead of ''healthy aging'' or ''aging well'' would be a delayed aging, postponed aging, slow aging, arrested aging, or negligible aging (senescence).
Because the reliability definition of biological aging is linked to health failures (adverse health outcomes, including death), aging without diseases is just as inconceivable as dying without death. Diseases and disabilities are an integral part (outcomes) of the aging process. Not every disease is related to aging, but every progression of disease with age has some relevance to aging: aging is a ''maturation'' of diseases with age.
Note that a system may have an aging behavior for one particular type of failure, but it may remain to be as good as new for some other type of failure. Thus the notion of aging is outcome-specific—it requires specifying for which particular type of failure (or group of failures) the system deteriorates. Consequently, the legitimate antiaging interventions may be outcome-specific too, and limited to postponing some specific adverse health outcomes. Aging is likely to be a summary term for many different processes leading to various types of degradation failures, and each of these processes deserves to be studied and prevented.
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