Bathtub Curve
Used in reliability engineering, it describes a particular form of the hazard function (a type of failure) comprising three distinct parts — the first part is a decreasing failure rate known as early failures, the second a constant failure rate, known as random failures, and the third is an increasing failure rate, known as wear-out failures.
Origin
The bathtub curve has no single inventor; its shape was recognized first in life insurance mortality data and later in engineering. The mathematical foundation arrived with Swedish engineer Waloddi Weibull's landmark 1951 paper on statistical failure distributions. The three-phase model gained currency in the 1960s as reliability engineering matured, popularized through U.S. military handbooks for electronics and aerospace systems developed under NASA's space programs.