Bath Tub Curve

The bathtub curve is widely used in reliability engineering, although the general concept is also applicable to humans. It describes a particular form of the hazard function which comprises three parts:The first part is a decreasing failure rate, known as early failures or infant mortality.The second part is a constant failure rate, known as random failures.The third part is an increasing failure rate, known as wear-out failures.
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failures rates over time fall into three distinct regions. The first region is referred to as infant mortality, or parts that fail at some relatively higher rate over a short period of time, due to manufacturing defects. Once the infant mortality is removed from the population, the useful life period is reached with a relatively lower failure rate. During the useful life failures are random in time. The final stage is wear out, the parts begin to wear out and fail at some relatively higher rate. If the failure rate of the parts versus time is plotted on a curve, the high failure rates initially and at end of life with relatively low failure rates during the middle period results in a bath tub shaped curve.