Theory of Constraints Handbook - James Cox Iii [681]
There are three different types of variability significantly affecting an organization.
Type 1—This occurs when most of the variability is within the task itself and not in the resource queue (see Fig. 35-5). The most significant known or anticipated variability will be in the work being performed in task A, B, and C. Remember in planning we are identifying what work must be done and the resources required within the tasks. This is not implying that in execution there will not be variability due to lack of resources, or set-up or set-down time. In fact, it is very highly likely that many of the tasks will be impacted by the required resources not being available. Conversely, some of the resources will spend time in the queue waiting for predecessor tasks to finish, thus allowing the successor task to start.
Type 2—This exists when the variability within the task itself is relatively low and most of the variability is in the queue. This assumes well-defined manufacturing processes and well-defined tasks. In Fig. 35-5, Tasks A, B, and C the variability is low because this particular work or something very similar has been done many times before. In companies using MRP or MRPII, the manufacturing routings are readily available and will be incorporated into the master schedule. The same can be said for the set up and set down; the required time is well known and variability is minimum.
Type 3—Occurs when the variability is in the demand pattern of material requirements. This can be within the company if the part is currently in inventory or is a component being manufactured internally. Sometimes the material is outsourced and must be delivered in time to support the company’s master schedule. This is further complicated by having to anticipate future market demand for all products, which of course determines what material is needed, the quantity, and precisely when it must be available.
Different Tools for Different Types of Variability
If there are three types of variability, this leads to a requirement for separate and distinct algorithms for planning, scheduling, and execution. The three commonly used algorithms are as follows:
Project Management—Type 1 variability, which relies heavily on the concept of critical path methodology and establishing well-defined relationships of the tasks. Once the tasks have been identified, the correct sequencing will yield the project network. This network becomes the schedule for managing the resources and executing the project. Again, in Fig. 35-5, the greatest uncertainty or variability is captured within the individual tasks. The project network schedule will not have any protection against variability in the resource queue. Typically, the amount of protection time for variability placed within the task is two or three times the actual productive time required.
Production Floor Scheduling—Type 2 variability, which relies on developing well-defined relationships of the tasks and identifying resources. This algorithm does not use the concept of critical path methodology. As shown in Fig. 35-6, Task A, B, and C and set up and set down have very little variability. This drives most of the variability into the resource queue. In fact, if one looks at the ratio between the times scheduled to accomplish all of the tasks in manufacturing of the individual product to the productive time (Fig. 35-5), which is the actual touch time needed, this confirms most of the time in the schedule is placed in the resource queue. It is not uncommon to schedule the manufacturing cycle time with 10, 20, or more times than the actual touch time (Schragenheim and Walsh, 2004).
FIGURE 35-6a Traditional project network with buffering within each task.
FIGURE 35-6b Critical chain project network with strategic time buffers.
Material Management and Inventory Control—Type 3 variability is managed by providing safety