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People and products are connected by the concept of the bathtub curve — the term describes the shape of the graph of the frequency of death or failure relative to the age of the person or item. The characteristic shape has high values at the beginning and end with a long trough in the middle. Three stages comprise the bathtub shape: the initial infant mortality stage, the useful life stage and the wear-out stage. The curve describes the characteristics of a population; it does not necessarily give information about a particular member of the sample.
In the first stage, a relatively high proportion of the people or items have problems that result in death or breakage. These are a result of initial flaws: in people, these can be birth defects or genetic abnormalities, while in products they are mistakes in manufacturing. As this infant mortality stage progresses, fewer cases occur because the most serious problems result in immediate failure or death.
After the frequency of death or failure stops decreasing, the infant mortality stage is over. At this point, the bathtub curve enters the useful life stage. Only healthy or sound individuals remain in this stage, and the problems that do occur arise from chance or accident. Values in this section are generally much lower than in the other two stages.
Finally, the occurrence of death or failure rises again as the people and items get older and begin to fall into bad health or break. These problems are a result of the person or item having completed a normal lifespan. The increase in problems completes the shape of the bathtub curve.
Policymakers and product engineers monitor the bathtub curve for similar reasons. Both want to elongate the useful life stage as much as possible, and they want to focus on reducing the instance of infant mortality. They can see the effects of their decisions in the way that the graph changes.
In human cases, people act against the causes of problems. They take actions such as educating mothers about prenatal care and making advances in medical technology to prevent infant death and to stave off the effects of old age in the general population. The result is a bathtub curve that has a longer trough with lower infant mortality, which means that more people survive to live long, healthy lives.
Product developers pair the search for causes with a burn-in approach. They stress the products before they are sent out to customers to ensure that only the strongest products survive to be sold. This helps them weed out products that have faults whose cause the developers have not yet identified.
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