The bathtub curve (or bathtub effect) describes the reliability of technology over time. In other words, the distribution of failures. The bathtub curve corresponds to the graphic representation of the (observed) failure rate of a technical device in the product life cycle.
This curve can be divided into three phases:
Phase 1 (early failure): Faulty equipment (e.g. defective design, production or materials) usually fails right at the start of operation.
Phase 2 (normal operation): occasional failures, mostly due to coincidences
Phase 3 (wear): Failures increase due to ageing and signs of wear.
The course of this composite graph reminds one of the shape of a bathtub in cross-section.