Читать книгу Intermittent Demand Forecasting - John E. Boylan - Страница 36

The design and construction of any physical product is captured in its bill of materials (BoM) which, as illustrated in Figure 2.1, shows the product's assemblies (middle level) and components (bottom level). Demand at the top level, corresponding to the end product, is defined as ‘independent’, whereas demand at lower levels in the hierarchy is dependent on demand at higher levels.

If a company has a ‘make to stock’ policy, then immediate availability and thus shipment of the end products (also known as “finished goods”) is promised to the customers. If a company operates a ‘make to order’ (MTO) policy, then immediate availability of the end products is not offered, but rather a delivery date is promised. In both policies, all the components and assemblies are produced or procured in time for the end product to reach the client by the promised time.

Figure 2.1 Bill of materials (BoM) example.

To meet delivery targets, a scheduling procedure is required and this is the backbone of the principal inventory methodology for dependent demand items, called material requirements planning (MRP) (see, for example, Ptak and Smith 2011). MRP will give a full BoM, showing all the elements needed for the final product, together with the required quantities. This is illustrated in Figure 2.1 for the example of a bicycle (end product, highlighted in bold type) comprising a frame, two wheels, and a seat (assemblies) and with each wheel comprising a wheel rim, 24 spokes, and a tyre (components).

MRP will give a full schedule of requirements, including quantities and dates by which all elements of the product must be produced or procured. In the example above, two wheel rims, 48 spokes, and two tyres would be required. Lead times for these components would also be specified, allowing all required delivery dates to be calculated. By following an MRP procedure, it is clear that if we know or can forecast the demand for the end product, then the requirements for all the lower levels can be calculated immediately.