In probability theory and statistics, the exponential distribution is a continuous probability distribution with the probability density function
The graph below shows the probability density function for λ equal to 0.5, 1.0, and 1.5:
The expected value and standard deviation of an exponential random variable are both 1/λ (and thus its variance is 1/λ2.)
The exponential distribution is used to model Poisson processes, which are situations in which an object initially in state A can change to state B with constant probability per unit time λ. The time at which the state actually changes is described by an exponential random variable with parameter λ. Therefore, the integral from 0 to T over f is the probability that the object is in state B at time T.
The exponential distribution may be viewed as a continuous counterpart of the geometric distribution, which describes the number of Bernoulli trials necessary for a discrete process to change state. In contrast, the exponential distribution describes the time for a continuous process to change state.
Examples of variables that are approximately exponentially distributed are: