We saw how processes in operations can range from producing a very high volume of products or services (for example, a food canning factory) to a very low volume (for example, major project consulting engineers). Also, they can range from producing a very low variety of products or services (for example, in an electricity utility) to a very high variety (as, for example, in an architects’ practice). Usually, the two dimensions of volume and variety go together. Low-volume operations processes frequently have a high form of products and services, and high-quantity operations approaches often have a narrow kind of products and services. Consequently, there may be a continuum from low volume and high variety thru to the high extent and occasional range, on which we can function operations.
Variations which derive from these common causes can never be entirely eliminated (although they can be reduced). For example, if a machine is filling boxes with rice, it will not place exactly the same weight of rice in every box it fills. When the filling machine is in a stable condition (that is, no exceptional factors are influencing its behaviour) each box could be weighed and a histogram of the weights could be built up.
The concept of variability is central to understanding the behaviour of queues. If there were no variability there would be no need for queues to occur because the capacity of a process could be relatively easily adjusted to match demand. For example, suppose one member of staff (a server) serves at bank counter customers who always arrive exactly every five minutes (i.e. 12 per hour). Also suppose that every customer takes exactly five minutes to be served, then because, the arrival rate is ? processing rate, and there is no variation.