One of the simplest and most common approximations is to assume a Poisson process for arrivals, which corresponds to exponential distribution of the inter-arriv

One of the simplest and most common approximations is to assume a Poisson process for arrivals, which corresponds to exponential distribution of the inter-arrival times. It's tempting to model work time with an exponential distribution too, and there's a lot of theory for this case. Unfortunately, exponential distribution has a mode = 0, so it's not a good choice for real life process times. It's more common to see distributions like lognormal, gamma, Weibull, loglogistic, beta, or others with a mode > 0.