8. Aggregate Planning, MRP and Enterprise Resource Planning

Introduction and Context

Operations management connects market demand with the resources, materials, people, equipment and information required to satisfy that demand. In logistics-intensive organizations, this connection is especially important because production decisions influence purchasing, inventory, warehousing, transportation, supplier relationships and customer service. A firm may have adequate production capacity but still fail to deliver because a critical component is unavailable. Conversely, it may hold excessive inventory because planning decisions were made independently by sales, production and procurement.

Aggregate planning, material requirements planning (MRP), capacity requirements planning (CRP), distribution requirements planning (DRP) and enterprise resource planning (ERP) are related mechanisms for coordinating these decisions. They operate at different levels and time horizons. Aggregate planning establishes broad production, workforce and inventory policies. The master production schedule (MPS) converts those policies into specific end-item commitments. MRP determines which components and raw materials are needed, in what quantities and when. CRP tests whether the required capacity is available. DRP extends planning across distribution networks, while ERP integrates these activities with finance, sales, procurement, human resources and logistics.

The central managerial challenge is not simply to produce a plan. It is to create a feasible, responsive and economically defensible plan that can be revised as demand, supply and capacity conditions change.

Learning Objectives

After studying this chapter, students should be able to:

1. Explain the purpose, scope and relationships among aggregate planning, MPS, MRP, CRP, DRP and ERP.

2. Develop and interpret an aggregate production plan using level, chase and hybrid strategies.

3. Translate an aggregate plan into a master production schedule.

4. Use bills of materials, inventory records and lead times to perform basic MRP calculations.

5. Explain capacity requirements planning and distribution requirements planning.

6. Evaluate the managerial, technological and organizational implications of ERP integration.

7. Identify planning risks, implementation challenges, ethical concerns and contemporary digital developments.

8. Recommend an integrated planning approach for a logistics or manufacturing organization.

Planning Across Time Horizons

Planning systems are commonly distinguished by their time horizon and level of detail. Strategic planning may address facility location, technology, supplier networks and long-term capacity over several years. Tactical planning generally covers months to approximately two years and includes aggregate production, workforce and inventory decisions. Operational planning covers weeks or days and specifies orders, materials, schedules, dispatching and shipments.

These levels are interdependent. A long-term decision to install a production line creates capacity that constrains the aggregate plan. The aggregate plan sets broad output and inventory targets. The MPS identifies particular products and quantities. MRP then explodes those products into component requirements. CRP evaluates whether labor and machine capacity can support the plan. DRP coordinates replenishment across warehouses and distribution centers. ERP provides a shared information environment in which these plans and transactions can be coordinated.

The planning hierarchy is therefore both vertical and horizontal. Vertically, decisions move from strategic intent to operational execution. Horizontally, departments must coordinate across sales, operations, procurement, logistics, finance and customer service. Poor integration at either level produces familiar symptoms: expediting, stockouts, excess inventory, unstable schedules, overtime and unreliable delivery promises.

Aggregate Planning: Converting Demand into a Feasible Operations Policy

Aggregate planning is the medium-term process of determining overall production, workforce, inventory, subcontracting and backlog levels for product families rather than individual stock-keeping units. A product family may consist of several models that consume similar resources or are measured in a common unit such as labor hours, tonnes, cases or standard units.

The aggregate plan normally begins with a forecast of demand by period. The planner then compares expected demand with available capacity and selects a policy for balancing them. The principal decision variables are:

• production quantity by period;
• workforce size and workforce changes;
• regular-time, overtime and subcontracted capacity;
• inventory carried between periods;
• planned backorders or lost sales; and
• use of existing or additional facilities and equipment.

The objective is usually to minimize relevant cost while satisfying service and policy requirements. A simplified aggregate-planning cost expression is:

Total cost = regular production cost + overtime cost + hiring and layoff cost + inventory holding cost + subcontracting cost + shortage or backorder cost.

Real decisions may also include environmental costs, labor agreements, quality effects, supplier risk and strategic considerations that are not captured fully by a simple cost function.

Demand and Capacity Inputs

Typical inputs include demand forecasts, beginning inventory, workforce levels, productivity rates, labor and material costs, available machine hours, storage limits, minimum workforce requirements and supplier capacity. Forecast uncertainty should be represented explicitly. A single forecast can create false precision; scenario ranges or probabilistic forecasts are often more useful for managerial decisions.

Capacity is not simply the number of machines installed. It depends on working time, staffing, maintenance, setup losses, quality yields, material availability and bottleneck resources. A plant with nominal capacity of 10,000 units per month may have effective capacity of only 8,200 units after accounting for downtime, changeovers and planned maintenance.

Level, Chase and Hybrid Strategies

A level strategy maintains a relatively stable workforce and production rate. Demand fluctuations are absorbed through inventory, backorders or a combination of both. The strategy supports workforce stability and can protect productivity, but it may create high inventory or service problems when demand is volatile.

A chase strategy adjusts production to demand by changing workforce levels, working hours, subcontracting or other capacity variables. It reduces finished-goods inventory but may lead to hiring and training costs, labor dissatisfaction, quality variation and loss of organizational knowledge.

A hybrid strategy combines policies. For example, a company may maintain a core workforce, use overtime during moderate peaks, subcontract specialized operations during extreme peaks and permit limited inventory variation. Hybrid strategies are common because they recognize that capacity levers have different costs, response times and risks.

Aggregate Planning Procedure

A practical planning process can follow these steps:

1. Define the planning horizon, product families, time buckets and performance objectives.

2. Prepare a demand forecast and identify uncertainty, seasonality and known events.

3. Establish beginning inventory, workforce, capacity and supply assumptions.

4. Generate alternative plans using level, chase or hybrid policies.

5. Calculate production, inventory, workforce and relevant costs for each period.

6. Test material, labor, equipment, storage and supplier feasibility.

7. Evaluate service, risk, sustainability and labor implications.

8. Select, approve and periodically revise the plan.

Quantitative tools include spreadsheets, linear programming, mixed-integer programming, simulation and optimization software. However, a mathematically economical plan is not automatically implementable. For example, a model may assume fractional workers, unlimited supplier flexibility or instantaneous hiring. Managerial judgment is needed to assess such assumptions.

From Aggregate Plan to Master Production Schedule

The master production schedule is the anticipated schedule for producing specific end items in identified quantities and time periods. It translates aggregate decisions into a form that sales, manufacturing, procurement and logistics can use. The MPS is more detailed than the aggregate plan but remains a managerial commitment rather than an unchangeable promise.

The MPS must reconcile three considerations: customer demand, available resources and inventory policy. It commonly includes forecast demand, customer orders, projected available balance, available-to-promise inventory and planned production quantities. The schedule is often divided into time fences. Within a firm or frozen zone, changes are restricted because materials and capacity have already been committed. In a slushy zone, changes are possible with authorization. In an open zone, the schedule can be modified more freely.

A basic projected available balance can be expressed as:

Projected available balance = previous balance + scheduled receipts − demand.

When the projected balance would fall below the required safety stock, a planned order receipt is scheduled. The timing of the planned order release is then determined by subtracting the manufacturing lead time. The MPS should be checked against rough-cut capacity planning, which evaluates key resources before detailed MRP is executed.

The MPS also supports customer order promising. Available-to-promise calculations estimate how much inventory or future production remains uncommitted to existing orders. This allows sales staff to provide delivery dates based on operational evidence rather than optimism.

Material Requirements Planning

MRP is a time-phased planning method for dependent-demand items. Dependent demand is derived from the demand for another item. If one finished product requires four wheels, demand for wheels depends on the planned production of the finished product. This differs from independent demand, such as customer demand for finished products, which is usually forecast directly.

MRP answers three questions:

1. What materials and components are required?

2. How many are required?

3. When must orders be released so that the items are available when needed?

MRP depends on three principal inputs:

• the MPS;
• the bill of materials; and
• inventory records.

Bills of Materials

A bill of materials (BOM) is a structured description of every component, subassembly, raw material and quantity required to make a parent item. A single-level BOM identifies immediate components. A multi-level or indented BOM shows the complete product structure and the relationship between levels.

For example, a distribution kit may require one finished kit. Each kit requires two cartons and one labeling set. Each labeling set requires one label roll and one ink cartridge. If the MPS calls for 100 kits, gross requirements are 200 cartons, 100 labeling sets, 100 label rolls and 100 ink cartridges, before considering inventory, scheduled receipts, scrap and lot-sizing rules.

BOM accuracy is essential. Incorrect quantities, obsolete engineering revisions, missing components or inappropriate units of measure will produce incorrect purchasing and production recommendations. Product lifecycle management systems and engineering change controls are therefore closely connected to MRP.

MRP Records and Netting Logic

An MRP record usually shows, by period, gross requirements, scheduled receipts, projected available balance, net requirements, planned order receipts and planned order releases. The logic is based on netting available supply against gross requirements.

A simplified calculation is:

Net requirement = gross requirement − available inventory − scheduled receipts + safety stock allowance, subject to the organization’s exact convention.

Suppose 80 units of a component are required in week 4. Beginning available inventory is 20 units, and 10 units are due to arrive in week 3. If a safety stock of 5 units must remain, the net requirement is 55 units. If supplier lead time is two weeks, the planned order release occurs in week 2. Lot-for-lot ordering would release 55 units. A fixed-order-quantity rule might require a larger quantity, such as 100 units, creating excess inventory but reducing ordering frequency.

Common lot-sizing approaches include lot-for-lot, fixed order quantity, economic order quantity, periodic order quantity and part-period balancing. The best choice depends on setup cost, supplier constraints, price breaks, perishability, storage cost and service requirements.

MRP Assumptions and Limitations

Traditional MRP assumes that the MPS, BOM, inventory data and lead times are sufficiently accurate. It may also assume that capacity is available unless a separate capacity check is performed. These assumptions explain why MRP can produce formally correct but operationally infeasible plans.

MRP is sensitive to data errors and planning instability. A small change in the MPS can create numerous changes in component orders, particularly in products with deep BOMs. This phenomenon is sometimes called nervousness. Time fences, order modifiers, safety stocks, firm planned orders and appropriate planning parameters can reduce unnecessary disruption.

MRP also does not automatically resolve strategic questions such as whether to make or buy, whether a supplier is financially reliable or whether a component design should be simplified. It provides planning signals; managers must interpret and govern them.

Capacity Requirements Planning and Distribution Requirements Planning

Capacity requirements planning translates planned order releases into requirements for work centers, labor groups, machines and other capacity resources. It uses routing data, which specifies the operations, sequence, setup time, run time and work center associated with an item. CRP compares required capacity with available capacity for each period.

When requirements exceed capacity, managers may revise the MPS, authorize overtime, subcontract work, add shifts, alter routings, move work to alternative resources or negotiate delivery dates. The important principle is that material feasibility and capacity feasibility are distinct. Having all components available does not mean that the organization can process them on time.

Distribution requirements planning applies time-phased planning logic to replenishment across a distribution network. It begins with demand or planned shipments at regional warehouses and calculates replenishment requirements from central warehouses, plants or suppliers. DRP can coordinate transportation loads, warehouse inventory and deployment decisions. It is particularly useful where multiple stocking points serve geographically dispersed demand.

DRP and MRP are conceptually similar but operate in different contexts. MRP plans dependent component demand within production. DRP plans inventory movement and replenishment across the distribution system. In an integrated system, the two are linked: the distribution plan affects the MPS, and the MPS affects material and capacity requirements.

ERP as an Integrating Architecture

Enterprise resource planning is an integrated information system that coordinates core organizational processes using a common database, shared master data and connected transaction flows. ERP commonly links sales and distribution, production, procurement, inventory, warehouse management, transportation, finance, maintenance, quality and human resources.

An order entered by a sales employee may reduce available-to-promise inventory, create a delivery requirement, affect the MPS, trigger MRP recommendations, generate purchase requisitions and produce accounting entries. The value of integration lies in the visibility and consistency of these relationships. Without integration, departments may maintain separate spreadsheets and databases that disagree about inventory, orders or customer commitments.

ERP implementation is not merely an information-technology installation. It changes processes, roles, performance measures and authority structures. Organizations must decide whether to adapt their processes to standard system practices, customize the system, or combine both approaches. Excessive customization increases cost and complicates upgrades; excessive standardization may ignore legitimate competitive or regulatory requirements.

Master Data and Governance

ERP performance depends on master data such as item numbers, units of measure, BOMs, routings, lead times, supplier records, locations, calendars and inventory policies. Data governance establishes ownership, validation, approval, version control and auditability. A sophisticated ERP system cannot compensate for inaccurate master data.

Governance also includes authorization controls. Users should have appropriate access to create suppliers, approve purchase orders, alter BOMs, release production orders or change customer credit limits. Segregation of duties reduces the risk of fraud and unauthorized transactions.

Digital Analytics and Automation

Contemporary planning environments increasingly combine ERP with advanced planning and scheduling, warehouse management, transportation management, Internet of Things devices, cloud platforms and analytics. Real-time inventory signals can improve replenishment visibility. Machine-learning models can support demand sensing, anomaly detection and lead-time estimation. Digital twins and simulation can test alternative capacity and network plans before implementation.

These tools should augment, not replace, professional judgment. Algorithms may reproduce historical bias, overreact to temporary demand changes or generate recommendations that are difficult to explain. Human oversight, data-quality monitoring and scenario analysis remain necessary, especially for high-impact decisions involving suppliers, workers and customer commitments.

Illustrative Case: Integrating Planning at Northstar Medical Devices

Background

Northstar Medical Devices manufactures diagnostic kits through a network consisting of one assembly plant, two regional distribution centers and approximately 60 approved suppliers. Demand is seasonal and affected by public-health programs. The company had an ERP system, but production, procurement and distribution teams used separate spreadsheets for short-term planning.

Situation and Problem

Northstar experienced frequent shortages of plastic housings and reagent cartridges even though total inventory value was rising. Sales frequently promised expedited deliveries without consulting production. The MPS changed several times each week, suppliers received conflicting order signals, and the distribution centers held safety stock independently. Management concluded that the problem was not simply insufficient inventory; it was a lack of synchronized planning.

Analysis

An audit found three major causes. First, the BOM for one kit had not been updated after a component substitution. Second, supplier lead times in the ERP system were based on historical assumptions rather than current performance. Third, the plant’s bottleneck sterilization process was omitted from rough-cut capacity checks. As a result, MRP recommendations were materially inaccurate, while the distribution centers placed duplicate replenishment orders.

Approach and Solution

Northstar established a cross-functional sales and operations planning meeting each month. Product families were forecast at the aggregate level, and a constrained MPS was approved for the next 12 weeks. A frozen period was introduced for the first two weeks, and changes required operations approval. Engineering and operations jointly reviewed BOM revisions, while procurement measured supplier lead-time variability rather than using a single average.

The company then activated CRP for the sterilization bottleneck and introduced DRP across the two distribution centers. Safety stock targets were differentiated by demand variability and service criticality. ERP dashboards displayed projected shortages, capacity overloads, overdue purchase orders and inventory by location. A weekly exception meeting focused only on material and capacity issues exceeding predefined thresholds.

Outcome

The integrated process reduced conflicting schedules and made shortages visible earlier. The company was able to distinguish genuine supply risk from duplicate or poorly timed orders. Inventory was not reduced indiscriminately; instead, it was repositioned toward critical components and regional demand points. Customer-service performance improved because delivery promises were based on the constrained MPS and available-to-promise information.

Learning Points

The case illustrates that ERP cannot create integration by itself. Integration requires common data, agreed planning rules, cross-functional governance and disciplined exception management. It also demonstrates that inventory reduction should not be treated as the sole objective. The appropriate aim is reliable service at an acceptable total cost and risk level.

Challenges, Risks and Common Mistakes

Several problems repeatedly undermine planning systems. Forecast error can produce either excess stock or shortages. Unstable schedules create nervousness throughout the BOM and strain suppliers. Inaccurate inventory records cause MRP to plan materials that are already available or to omit materials that are missing. Incorrect lead times create systematically early or late orders. Ignoring scrap, yield loss, minimum order quantities and supplier calendars produces unrealistic requirements.

Organizations also make structural mistakes. They may aggregate products that do not share resources, use financial forecasts without operational validation, or treat the MPS as a sales promise rather than a constrained plan. Another common error is to measure purchasing performance only through price reduction, encouraging large orders that increase inventory and obsolescence.

ERP implementations face additional risks: unclear process ownership, inadequate training, poor data migration, excessive customization, weak executive sponsorship and failure to redesign obsolete processes. Cybersecurity is also significant because an ERP system contains commercially sensitive, financial and operational information. Unauthorized alteration of a BOM, supplier bank account or shipment record can cause serious disruption.

Ethical, Governance and Professional Considerations

Planning decisions affect workers, suppliers, customers and communities. A chase strategy may improve financial results while imposing unreasonable employment instability. Subcontracting may transfer safety, environmental or labor risks to less visible suppliers. Inventory policies for medical, food or emergency products have ethical consequences when shortages affect vulnerable users.

Professionals should maintain transparent assumptions, distinguish forecast from commitment and avoid manipulating inventory or delivery metrics to meet short-term targets. Governance should include audit trails, approval controls, supplier due diligence, privacy protection and business-continuity planning. Where AI supports forecasts or allocation decisions, organizations should monitor bias, explainability, data provenance and human accountability.

Best Practices for Integrated Planning

Effective organizations generally adopt the following practices:

• Maintain a formal sales and operations planning cycle linking demand, supply, finance and risk.
• Use product-family plans for aggregate decisions and disaggregate carefully into a controlled MPS.
• Establish frozen and flexible planning zones.
• Treat BOMs, routings, lead times and inventory balances as governed master data.
• Perform both material and capacity checks before confirming commitments.
• Use exception-based management rather than reviewing every transaction manually.
• Coordinate MRP with DRP, procurement, warehouse and transportation plans.
• Measure service, inventory, schedule stability, capacity utilization, forecast error and total cost together.
• Train users in process logic, not merely software navigation.
• Rehearse disruption scenarios involving supplier failure, demand surges and transportation interruptions.

Current Developments and Emerging Directions

Planning systems are moving toward integrated business planning, in which financial, commercial, operational and sustainability objectives are evaluated together. Cloud-based platforms make collaboration with suppliers and logistics partners easier, although they increase dependence on connectivity, cybersecurity and vendor governance. Event-driven planning uses near-real-time signals such as point-of-sale data, shipment status and machine condition.

Advanced planning systems increasingly support probabilistic forecasts, scenario comparison and constraint-based scheduling. Sustainability considerations are also being incorporated into planning through carbon-aware sourcing, transport-mode selection, energy constraints and circular-material flows. The emerging direction is not simply faster automation, but more transparent and resilient decision-making across the entire supply network.

Chapter Summary

Aggregate planning establishes medium-term policies for production, workforce, inventory, subcontracting and service. Level, chase and hybrid strategies balance demand and capacity in different ways. The master production schedule converts aggregate plans into specific end-item commitments and must be constrained by capacity and material realities. MRP uses the MPS, BOM and inventory records to calculate time-phased component requirements, while CRP tests whether the required labor and equipment are available. DRP extends planning across distribution locations. ERP integrates these processes with sales, procurement, logistics, finance and other organizational functions. Successful implementation depends on accurate master data, disciplined governance, cross-functional participation and appropriate use of digital technologies. The central lesson is that planning is an integrated managerial process, not merely a software calculation.

Key Terms

Aggregate planning: Medium-term planning of production, workforce, inventory and related resources for product families.

Available-to-promise: The quantity that can be committed to new customer orders after existing commitments are considered.

Bill of materials (BOM): A structured list of components, materials and quantities required to produce a parent item.

Capacity requirements planning (CRP): A process for comparing time-phased capacity requirements with available work-center resources.

Chase strategy: An aggregate-planning policy that adjusts production capacity to follow demand.

Dependent demand: Demand calculated from the requirements of another item or parent product.

Distribution requirements planning (DRP): Time-phased planning of replenishment across distribution locations.

Enterprise resource planning (ERP): An integrated system coordinating organizational processes through shared data and connected transactions.

Frozen zone: A period in the planning horizon during which schedule changes are restricted.

Independent demand: Demand for an item that is determined directly by customers or the market.

Lot sizing: The method used to determine the quantity released in a planned order.

Master production schedule (MPS): A time-phased plan specifying production quantities for particular end items.

Material requirements planning (MRP): A system that calculates the quantities and timing of dependent-demand materials.

Net requirement: The requirement remaining after available inventory and scheduled receipts are considered.

Projected available balance: Expected inventory remaining after planned receipts and demand in a period.

Rough-cut capacity planning: A preliminary test of whether major resources can support the MPS.

Sales and operations planning: A cross-functional process for reconciling demand, supply, finance and strategic objectives.

Time fence: A boundary that defines the degree of schedule-change flexibility in a planning period.

Review Questions

Short Answer Questions

1. What is the purpose of aggregate planning, and why is it normally performed for product families?

2. Distinguish between a level strategy and a chase strategy.

3. Identify the three principal inputs required by MRP.

4. What is the role of CRP in relation to MRP?

5. Explain why master data governance is important in an ERP environment.

Long Answer Questions

1. Explain the planning hierarchy from aggregate planning through MPS, MRP, CRP and DRP.

2. Discuss the advantages and risks of level, chase and hybrid aggregate-planning strategies.

3. Describe the logic of MRP using a BOM, inventory records, lead times and lot-sizing rules.

4. Explain how ERP integrates operational planning with procurement, logistics, sales and finance.

5. Evaluate the organizational and governance challenges of implementing an integrated planning system.

Analytical / Application Questions

1. A company forecasts demand of 1,000, 1,400 and 900 units over three months. Beginning inventory is 200 units, and the firm wants to finish each month with at least 100 units. Compare the likely operational consequences of a level strategy and a chase strategy.

2. A product requires two components of A and three components of B. The MPS calls for 500 products in week 5. Inventory includes 300 units of A and 200 units of B, with no scheduled receipts. Calculate the basic net requirements before safety stock and lot-sizing considerations.

3. A distribution center repeatedly orders stock even though the plant reports adequate inventory. Diagnose possible failures in the MRP-DRP-ERP connection and recommend a corrective action plan.

22. Multiple Choice Questions with Answers

Question 1

Which planning activity primarily determines workforce, inventory and production levels for product families over a medium-term horizon?

A. Aggregate planning

B. Dispatching

C. Cycle counting

D. Route sequencing

Correct Answer: A. Aggregate planning

Question 2

A firm maintains a stable workforce and production rate while using inventory to absorb seasonal demand. Which strategy is it using?

A. Chase strategy

B. Level strategy

C. Zero-inventory strategy

D. Expediting strategy

Correct Answer: B. Level strategy

Question 3

Which combination represents the principal inputs to a material requirements planning system?

A. MPS, BOM and inventory records

B. Payroll, routing guide and sales invoices

C. Supplier contracts, tax records and warehouse maps

D. Transport rates, customer credit and maintenance logs

Correct Answer: A. MPS, BOM and inventory records

Question 4

A finished product requires four units of a component. If the MPS calls for 300 finished products, what is the component's gross requirement before inventory and scheduled receipts are considered?

A. 75 units

B. 296 units

C. 304 units

D. 1,200 units

Correct Answer: D. 1,200 units

Question 5

What is the main purpose of a time fence in an MPS?

A. To eliminate all forecast error

B. To restrict disruptive changes near-term

C. To increase the number of product designs

D. To replace capacity planning

Correct Answer: B. To restrict disruptive changes near-term

Question 6

MRP recommends 500 units of a component for week 6, and the supplier lead time is three weeks. Assuming no other constraints, when should the planned order release occur?

A. Week 2

B. Week 3

C. Week 5

D. Week 9

Correct Answer: B. Week 3

Question 7

Which statement best distinguishes CRP from MRP?

A. CRP plans customer demand, while MRP plans finance

B. CRP checks resource capacity, while MRP calculates material requirements

C. CRP manages transportation, while MRP manages payroll

D. CRP creates BOMs, while MRP creates supplier contracts

Correct Answer: B. CRP checks resource capacity, while MRP calculates material requirements

Question 8

Distribution requirements planning is primarily concerned with:

A. Replenishment across distribution locations

B. Hiring and training production workers

C. Designing engineering drawings

D. Calculating employee benefits

Correct Answer: A. Replenishment across distribution locations

Question 9

A company has accurate software but obsolete BOMs and incorrect lead times. What is the most likely result?

A. Reliable plans because ERP corrects all data errors

B. Inaccurate material and capacity recommendations

C. Automatic elimination of demand variability

D. Higher capacity without investment

Correct Answer: B. Inaccurate material and capacity recommendations

Question 10

Which practice best supports successful ERP-enabled planning?

A. Allowing every department to maintain separate definitions of inventory

B. Changing the MPS freely within the frozen zone

C. Governing master data and linking cross-functional planning processes

D. Measuring purchasing only by the lowest unit price

Correct Answer: C. Governing master data and linking cross-functional planning processes