Distributions of time-to-failure (which may be called life distributions, rightly so in the case of a non-repairable, continuous-duty device or unit) discussed in the previous chapter assume (indirectly) only two states of functioning of a device, viz. failed or functioning. No differentiation was attempted among different possible states of being functional, say, with different associated levels of performance and failure times could be exactly observed without any error. Real-life situations exist where we have to recognize multiple states of functioning as also where the failure time of a device can be subject to error of observation/measurement. And, in the latter case which can complicate analysis even in the first case, attempts have been made to make use of the ‘fuzziness’ concept, with appropriate choice of membership functions.