It is not possible to predict with any precision or certainty for how long a specific power supply will operate or after how many hours it will fail. However, the standard practice for electronic and mechanical components and systems is to use probability analysis to determine, with reasonable confidence, the expected lifetime or likelihood of failure. The failure rate for a component or system is usually expressed as a frequency, e.g. failures per hour, and usually denoted by the symbol λ. The actual failure rate however typically varies over the life of equipment due to factors that can be attributed to ‘early life failures’, ‘random failures’ or ‘wear out failures’. The cumulative characteristic that results from this is commonly referred to as the ‘bathtub curve’, as shown on this slide. A supply's reliability is a function of multiple factors: a solid, conservative design with adequate margins, quality components with suitable ratings, thermal considerations with necessary derating, and a consistent manufacturing process. Reliability, or the probability of a component not failing before a given time, can be calculated with the following formula: R(t) = e-λt.