There are two causes of variability in processes. The first type is the random factors that are inherent to processes and very difficult or expensive to control or eliminate. They are beyond the operative’s control and include factors such as the number of customer’s orders per day or slower deliveries caused by heavy rain. The other type of variability is due to causes that can be identified and are easier to control (assignable causes). Although both causes may always be present in production systems, it is possible to have procedures in place aimed at reduce or avoid their occurrence.