Material Requirements Planning (MRP)

Related Terms: Enterprise Resource Planning; Inventory Control Systems

Material requirements planning (MRP) is a computer-based inventory management system designed to assist production managers in scheduling and placing orders for items of dependent demand. Dependent demand items are components of finished goods—such as raw materials, component parts, and subassemblies—for which the amount of inventory needed depends on the level of production of the final product. For example, in a plant that manufactured bicycles, dependent demand inventory items might include aluminum, tires, seats, and bike chains.

The first MRP systems of inventory management evolved in the 1940s and 1950s. They used mainframe computers to explode information from a bill of materials for a certain finished product into a production and purchasing plan for components. Before long, MRP was expanded to include information feedback loops so that production personnel could change and update the inputs into the system as needed. The next generation of MRP, known as manufacturing resources planning or MRP II, also incorporated marketing, finance, accounting, engineering, and human resources aspects into the planning process. A related concept that expands on MRP is enterprise resources planning (ERP), which uses computer technology to link the various functional areas across an entire business enterprise.

MRP works backward from a production plan for finished goods to develop requirements for components and raw materials. MRP begins with a schedule for finished goods that is converted into a schedule of requirements for the subassemblies, the component parts, and the raw materials needed to produce the final product within the established schedule. MRP is designed to answer three questions: what is needed? how much is needed? and when is it needed?"

MRP breaks down inventory requirements into planning periods so that production can be completed in a timely manner while inventory levels—and related carrying costs—are kept to a minimum. Implemented and used properly, it can help production managers plan for capacity needs and allocate production time. But MRP systems can be time consuming and costly to implement, which may put them out of range for some small businesses. In addition, the information that comes out of an MRP system is only as good as the information that goes into it. Companies must maintain current and accurate bills of materials, part numbers, and inventory records if they are to realize the potential benefits of MRP.

MRP INPUTS

The information input into MRP systems comes from three main sources: a bill of materials, a master schedule, and an inventory records file. The bill of materials is a listing of all the raw materials, component parts, subassemblies, and assemblies required to produce one unit of a specific finished product. Each different product made by a given manufacturer will have its own separate bill of materials. The bill of materials is arranged in a hierarchy, so that managers can see what materials are needed to complete each level of production. MRP uses the bill of materials to determine the quantity of each component that is needed to produce a certain number of finished products. From this quantity, the system subtracts the quantity of that item already in inventory to determine order requirements.

The master schedule outlines the anticipated production activities of the plant. Developed using both internal forecasts and external orders, it states the quantity of each product that will be manufactured and the time frame in which they will be needed. The master schedule separates the planning horizon into time "buckets," which are usually calendar weeks. The schedule must cover a time frame long enough to produce the final product. This total production time is equal to the sum of the lead times of all the related fabrication and assembly operations. It is important to note that master schedules are often generated according to demand and without regard to capacity. An MRP system cannot tell in advance if a schedule is not feasible, so managers may have to run several possibilities through the system before they find one that works.

The inventory records file provides an accounting of how much inventory is already on hand or on order, and thus should be subtracted from the material requirements. The inventory records file is used to track information on the status of each item by time period. This includes gross requirements, scheduled receipts, and the expected amount on hand. It includes other details for each item as well, like the supplier, the lead-time, and the lot size.

MRP PROCESSING

Using information culled from the bill of materials, master schedule, and inventory records file, an MRP system determines the net requirements for raw materials, component parts, and subassemblies for each period on the planning horizon. MRP processing first determines gross material requirements, then subtracts out the inventory on hand and adds back in the safety stock in order to compute the net requirements.

The main outputs from MRP include three primary reports and three secondary reports. The primary reports consist of: planned order schedules, which outline the quantity and timing of future material orders; order releases, which authorize orders to be made; and changes to planned orders, which might include cancellations or revisions of the quantity or time frame. The secondary reports generated by MRP include: performance control reports, which are used to track problems like missed delivery dates and stock outs in order to evaluate system performance; planning reports, which can be used in forecasting future inventory requirements; and exception reports, which call managers' attention to major problems like late orders or excessive scrap rates.

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