Many probability distributions can be used to model the failure distribution. A common model is the exponential failure distribution,
which is based on the exponential density function. The hazard rate function for this is:
Thus, for an exponential failure distribution, the hazard rate is a constant with respect to time (that is, the distribution is "memory-less"). For other distributions, such as a Weibull distribution or a log-normal distribution, the hazard function may not be constant with respect to time. For some such as the deterministic distribution it is monotonic increasing (analogous to "wearing out"), for others such as the Pareto distribution it is monotonic decreasing (analogous to "burning in"), while for many it is not monotonic.
Use Calculations section in Statistical Distributions for calculate Exponential values:
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