Dependent-Demand Planning: MRP and DRP

Introduction

Inventory decisions differ fundamentally according to the nature of demand. Independent demand arises outside the production system—for example, customer demand for finished products. Dependent demand is derived from the requirements of another item. If one bicycle requires two wheels, the demand for wheels depends on the planned production of bicycles. This relationship permits dependent-demand items to be planned through calculation rather than forecast independently.

Material Requirements Planning (MRP) converts a master production schedule into time-phased requirements for components, subassemblies and purchased materials. Distribution Requirements Planning (DRP) applies similar logic to the movement and replenishment of finished goods through a distribution network. Together, MRP and DRP connect market demand, production, procurement, inventory and logistics.

MRP is not merely a software report. It is a planning logic whose quality depends on accurate bills of material, reliable inventory records, credible lead times, disciplined lot-sizing and a feasible master schedule. At postgraduate level, the important managerial issue is not simply how to calculate planned orders, but how to govern the assumptions and feedback mechanisms on which those calculations depend.

Learning objectives

By the end of this chapter, the reader should be able to:

• distinguish independent demand from dependent demand;
• explain the structure and data requirements of MRP and DRP;
• use a bill of materials, inventory record and master production schedule to calculate gross requirements, net requirements, planned receipts and planned order releases;
• compare lot-for-lot, fixed-order-quantity, periodic-order-quantity and economic lot-sizing approaches;
• apply lead-time offsetting and recognize capacity implications;
• explain nervousness, rescheduling and the role of planning time fences;
• design a basic DRP schedule for a multi-echelon distribution system;
• evaluate implementation risks, governance requirements and contemporary digital developments.

1. Dependent demand and the planning hierarchy

Independent demand is determined by external customers or markets and is normally estimated using forecasts, orders or probabilistic models. Dependent demand is calculated from the demand for a parent item. It is therefore usually deterministic once the parent schedule, product structure and inventory policies are specified.

For a product structure, if each parent requires q units of a component and the parent has a planned order release of P, the component gross requirement is:

Here, GR is the component gross requirement in units per period, q is the usage quantity per parent, and P is the parent planned order release. The relationship assumes that all parent units have the same component structure and that scrap, yield loss and engineering changes are either absent or explicitly incorporated.

MRP normally operates within a hierarchy:

1. The sales and operations plan establishes aggregate output and resource direction.
2. The master production schedule (MPS) specifies end-item quantities by time period.
3. The bill of materials (BOM) translates end-item quantities into component relationships.
4. MRP calculates component and material replenishment orders.
5. Capacity requirements planning and supplier scheduling test feasibility.
6. Execution systems report completions, shortages, changes and exceptions.

The MPS is therefore both a demand statement and a control boundary. If it is changed frequently without considering capacity, suppliers or customer priorities, the MRP output becomes unstable.

2. The architecture and data of MRP

2.1 Bill of materials

A BOM is a structured description of the materials and subassemblies required to make one unit of a parent. It normally contains the item identifier, quantity per parent, unit of measure, effective dates, engineering revision and sometimes scrap or yield factors. A single-level BOM lists immediate components; a multi-level BOM shows the complete product structure.

For example, a kit has one frame, two wheels and one brake set. Each wheel requires one hub and twenty spokes. The demand for hubs is not calculated directly from final bicycle demand alone; it is derived through the wheel level. This is called requirements explosion.

BOM accuracy is a governance issue. An incorrect quantity, obsolete revision or missing phantom assembly can create systematic shortages or excess inventory. Engineering change control must therefore be integrated with planning dates.

2.2 Inventory records

For every planned item, the inventory record generally contains:

• gross requirements;
• scheduled receipts already ordered or released;
• projected on-hand balance;
• safety stock, if applicable;
• lot-sizing rule;
• lead time;
• planned order receipts;
• planned order releases;
• order status and exception messages.

The record must distinguish physical inventory from available inventory. Allocated stock, quality-hold stock, safety stock and rejected material may not be usable for a particular requirement. Data integrity is especially important because MRP is recursive: an error at a parent level propagates to lower levels.

2.3 Time buckets and planning horizon

MRP commonly uses weekly buckets, although daily or monthly buckets may be appropriate in particular environments. The planning horizon should cover at least the cumulative lead time of the relevant products and preferably extend far enough to reveal capacity and supplier implications. A frozen near-term zone may coexist with a flexible future zone.

2.4 The master production schedule

The MPS states when end items are expected to be completed. It should be based on confirmed orders, forecast demand, available inventory, service objectives and resource constraints. The MPS is not identical to a sales forecast: it is an operational commitment or proposal to produce specific quantities at specified times.

3. MRP calculation logic

The standard MRP record proceeds across time periods. The principal calculations are as follows.

Projected available balance before a planned receipt is:

where PAB_t is projected available balance at the end of period t, SR_t is scheduled receipt, PORc_t is planned order receipt, and GR_t is gross requirement. The opening balance is PAB₀. If safety stock is required, the balance should not normally fall below the specified safety-stock level.

Net requirement can be expressed as:

where NR_t is net requirement and SS is required safety stock. This formulation assumes that the previous projected balance and scheduled receipts are available to satisfy the period requirement. Different software systems may display safety stock and timing conventions differently, so the planner must verify the system definition.

The planned order receipt is the net requirement adjusted by the lot-sizing rule:

where L(·) is the selected lot-sizing function. Under lot-for-lot, L(NR_t) = NR_t. Under a fixed lot size Q, a positive requirement is rounded upward to a multiple of Q:

The ceiling function ensures that the order is not smaller than the requirement. This creates residual inventory, which must be carried into later periods.

Worked example: single-level MRP

An item has opening inventory of 40 units and no safety stock. Its gross requirements over four weeks are 30, 50, 20 and 60 units. A scheduled receipt of 40 units arrives in week 2. Lead time is one week and the lot-sizing rule is lot-for-lot.

In week 1, 40 − 30 = 10 units remain. In week 2, the balance becomes 10 + 40 − 50 = 0. Week 3 requires 20 units, so a planned receipt of 10 is needed because the available balance before the planned receipt is zero but the table’s opening convention shows the planned receipt calculated after accounting for the period requirement; equivalently, beginning inventory for week 3 is zero and the full requirement would be 20. To avoid ambiguity, the correct week-3 net requirement is 20, not 10. The corrected schedule is therefore:

This correction illustrates a practical lesson: MRP tables must state whether netting is performed against beginning balance or against balance after the current period’s gross requirement. The standard convention used here is to calculate the receipt needed so that ending projected balance meets the requirement. Under that convention, week 3 requires 20 units.

For a multi-level BOM, the planned order releases of a parent—not its gross requirements—normally generate the gross requirements of its components. If one parent requires three components and 100 parent units are released in week 5, each component has a week-5 gross requirement of 300 units, subject to scrap and yield adjustments.

4. Lot-sizing decisions

Lot sizing balances ordering or setup cost against holding cost, capacity, obsolescence and service risk. No single rule is universally optimal.

Lot-for-lot

Lot-for-lot orders exactly the calculated net requirement. It minimizes projected residual inventory and is attractive for expensive, perishable or customized items. It may produce many orders and setups, particularly when demand is intermittent.

Fixed order quantity

A fixed quantity may be imposed by supplier packaging, process economics, minimum order quantities or production batch requirements. It reduces order frequency but creates inventory peaks and may increase obsolescence.

Periodic order quantity

Periodic order quantity covers requirements for a selected number of periods. It can reduce administrative effort and stabilize releases, but it may hold more inventory than lot-for-lot.

Economic order quantity

For relatively stable independent demand, the classical economic order quantity is:

where Q* is order quantity in units, D is annual demand in units per year, S is ordering or setup cost per order, and H is annual holding cost per unit. The model assumes constant demand, instantaneous replenishment, no shortages, constant ordering and holding costs, and no quantity discounts. These assumptions are often unsuitable for strongly time-phased dependent demand; consequently, EOQ should not be applied mechanically within MRP.

Part-period balancing and least unit cost

Dynamic lot-sizing methods compare the marginal holding cost of carrying material forward with the cost of placing another order. Part-period balancing seeks a lot whose accumulated holding cost approximates the setup cost. Least unit cost selects the grouping with the lowest average cost per unit. These methods can be useful where demand varies by period, but they remain sensitive to forecast changes and cost estimates.

Managers should evaluate total relevant cost, not merely purchase price. Relevant considerations include setup, ordering, transport, storage, quality inspection, working capital, obsolescence, disruption exposure and supplier flexibility.

5. Lead-time offsetting and capacity links

If a planned receipt is needed in period t and the item’s lead time is ℓ periods, the corresponding planned release is normally scheduled in period:

The calculation assumes that lead time is measured consistently with the planning buckets and that capacity is available. In practice, manufacturing lead time may include queue, setup, processing, move and inspection time. Purchasing lead time may include approval, supplier processing, transit, receiving and inspection.

A planned order is therefore not automatically feasible. Capacity Requirements Planning (CRP) converts planned orders into work-centre loads. If load exceeds available capacity, management may revise the MPS, add shifts, subcontract, alter routings, split lots, use alternate resources or change due dates.

Capacity is often evaluated using:

This ratio is meaningful only when numerator and denominator use the same capacity unit and period. A utilization above 100% indicates overload under the stated assumptions; it does not by itself identify the best corrective action.

MRP II extends material planning by integrating purchasing, production, capacity, finance and performance feedback. Modern enterprise resource planning systems implement this integration, but software integration does not eliminate the need for managerial judgment.

6. Nervousness, stability and control

MRP nervousness is excessive change in planned orders caused by revised demand, late receipts, inventory-record errors, lead-time changes or changes in the MPS. Symptoms include repeated order cancellations, date changes, expediting, supplier frustration and unstable shop-floor priorities.

Nervousness is not always bad. A schedule should respond to genuine changes in customer requirements or supply risk. The problem is uncontrolled change whose operational cost exceeds its benefit.

Important stabilizing mechanisms include:

• time fences: a frozen zone where changes require authorization, a slushy zone with limited changes, and a liquid zone where changes are relatively easy;
• planning calendars: common definitions of working days, holidays and cut-off times;
• firm planned orders: orders protected from automatic rescheduling;
• exception messages: attention directed to shortages, lateness, excess, invalid dates and capacity overloads;
• safety lead time or stock: protection against variability, used selectively rather than as a substitute for process control;
• pegging: tracing a component requirement back to the parent order or customer demand that caused it.

The bullwhip effect may be amplified when every echelon reacts independently to small changes. Shared data, stable policies and collaborative planning reduce such amplification.

7. Distribution Requirements Planning

DRP is the time-phased planning of replenishment for finished goods or service parts across a distribution network. It applies the same basic logic as MRP but usually works from customer demand, forecasts, dependent replenishment demand and transport lead times rather than a manufacturing BOM.

A distribution network may contain a central plant, regional distribution centres, local warehouses and customers. DRP determines when and how much each stocking location should receive. The parent of a distribution-centre requirement is often the supplying location, so a planned shipment from the central warehouse becomes a gross requirement for the supplying warehouse or plant.

A DRP record typically contains:

• forecast and customer orders;
• beginning available inventory;
• scheduled receipts or in-transit shipments;
• safety stock and service policy;
• gross requirements;
• projected available balance;
• planned order receipts;
• planned order releases;
• transport and handling lead times.

The same netting relationship applies:

However, distribution planning must represent shipment timing accurately. If a regional warehouse needs a receipt in week 6 and transportation takes two weeks, the supplying location’s shipment release is planned for week 4. If the supplying location itself requires one week to prepare the shipment, its replenishment receipt may need to arrive in week 3.

DRP differs from simple reorder-point control because it is time-phased and network-oriented. Reorder-point systems respond mainly to inventory position and uncertainty; DRP coordinates future requirements across locations and can expose upstream consequences of downstream promotions.

Illustrative DRP schedule

A regional distribution centre begins with 80 units. Demand is 30 units in week 1, 50 in week 2 and 40 in week 3. A scheduled receipt of 40 units arrives in week 2. Safety stock is 20 units. Under lot-for-lot planning, the centre requires 20 additional units in week 3 to finish at safety stock, assuming no later demand is included.

The release shown in week 1 would be required for a week-3 receipt if the two-week lead time is measured from release to arrival. If the schedule is being prepared at the start of week 1 and releases cannot be made before that point, the requirement is already late or must be covered by an emergency shipment. This is why DRP must be integrated with transportation calendars and visibility of in-transit inventory.

8. Integrated MRP–DRP planning

MRP and DRP should not be treated as isolated modules. Customer demand creates distribution requirements; distribution requirements influence the MPS; the MPS drives component requirements; supplier and capacity constraints feed back into available-to-promise dates.

Figure: Integrated relationship between market demand, DRP, the MPS, MRP and supply execution.

The integration point is often the distribution plan’s requirement for finished goods. If regional warehouses require more product than the plant can make, the issue must be resolved through allocation, revised priorities, additional capacity, alternative sourcing or customer-date negotiation. Treating DRP as an automatic entitlement can create impossible production schedules.

9. Applied case: illustrative medical-device network

Background

A fictional medical-device manufacturer produces infusion controllers at a central plant and distributes them through two regional warehouses. Each controller requires one control board, one casing and one tested battery pack. The plant has a two-week manufacturing lead time. The control board has a four-week supplier lead time and a minimum order quantity of 200 units. Regional transport takes one week.

Problem

A hospital contract increases demand by 120 controllers in week 8. The eastern warehouse has 40 units available and a safety stock target of 20. The central plant has 60 finished units and 100 control boards. The MPS had previously planned only 80 controllers for week 8.

Analysis

The eastern warehouse requires 120 units. Its available inventory can cover 20 units above safety stock, leaving a replenishment requirement of 100 units, subject to the timing of other demand. DRP therefore creates a planned receipt of 100 controllers in week 8 and a shipment release in week 7. The plant must complete those units by week 7 or earlier, depending on dispatch and handling conventions.

The revised MPS increases the week-8 requirement from 80 to 180 units if the original 80 remains valid. MRP then creates additional requirements for 180 control boards, 180 casings and 180 battery packs, before considering existing inventory. The control-board shortage is particularly serious: 100 on hand plus a 200-unit minimum supplier lot can cover the requirement, but the order must be released at least four weeks before receipt. If the decision is made in week 5, normal supply cannot arrive before week 9 unless an expedited supplier arrangement is available.

Approach and solution

The company should first validate whether the warehouse requirement is a firm hospital commitment or an unconfirmed forecast. It should then peg the DRP requirement to the hospital order, establish an authorized priority, check the plant’s capacity, and evaluate alternatives: reallocating controllers from the other region, using finished-goods inventory, expediting control boards, approving a substitute component, or negotiating a staged delivery.

The minimum order of 200 boards may create 20 units of excess after satisfying the incremental 180-unit schedule, before accounting for existing stock. That excess may be acceptable because the boards are common and non-obsolescent; it would be less acceptable for a unique, regulated component.

Outcome

Assuming capacity and expedited procurement are approved, the company can protect the hospital commitment while maintaining traceability of the component revision and supplier approval. The MPS, DRP and purchase orders must be frozen through the agreed near-term time fence. The excess boards should be recorded as projected inventory, not ignored as an unplanned surplus.

Learning points

The case demonstrates that a customer requirement propagates through distribution, production and procurement. It also shows why MRP output cannot be accepted without capacity, lead-time and engineering validation. A mathematically correct plan may still be infeasible if a critical supplier cannot respond.

10. Limitations, governance and contemporary practice

MRP assumes that its inputs are sufficiently accurate. Common limitations include uncertain demand, variable lead times, inaccurate inventory balances, unrecorded scrap, inaccurate BOMs, finite capacity, supplier unreliability and excessive system nervousness. Safety stock protects against some uncertainty but does not repair poor master data.

Governance should assign ownership for item masters, BOM revisions, lead times, supplier calendars, inventory transactions and exception approval. Audit trails are important where materials affect product safety, regulatory compliance or traceability. In regulated sectors, a substituted component may require formal validation; planning convenience cannot override quality or regulatory controls.

Contemporary systems use cloud ERP, supplier portals, advanced planning and optimization, Internet of Things data, digital twins and machine-learning forecasts. AI can identify anomalous lead times, predict supplier delays, classify exception messages and improve demand sensing. Nevertheless, AI-generated recommendations require explainability, data-quality controls, human authorization and protection against feedback loops. A model trained on historically expedites may learn to recommend more expediting rather than addressing root causes.

Best practice includes:

• maintain one controlled item and BOM master;
• measure inventory-record accuracy and BOM accuracy separately;
• review lead times using actual performance, not outdated standards;
• use exception-based management rather than manually inspecting every line;
• integrate MRP with CRP, procurement, quality and logistics;
• define time fences and authority for changing firm orders;
• use pegging to understand the business consequence of shortages;
• segment items by value, criticality, variability and supply risk;
• test plans with scenarios for demand, capacity and supplier disruption;
• monitor outcomes such as service level, schedule adherence, inventory turns, expedites and obsolete stock.

Chapter Summary

Dependent-demand planning calculates component and distribution requirements from higher-level schedules. MRP begins with the MPS, explodes the BOM, nets requirements against inventory and scheduled receipts, applies lot-sizing, and offsets planned receipts by lead time to create releases. DRP uses comparable time-phased logic to replenish warehouses and coordinate a distribution network.

The quality of MRP or DRP depends on accurate master data, realistic lead times, disciplined inventory transactions and a feasible MPS. Lot-for-lot minimizes residual inventory but can increase setups; fixed and dynamic lot-sizing methods trade ordering cost against holding and obsolescence. Capacity requirements planning is necessary because planned orders are not automatically executable. Nervousness should be controlled through time fences, firm orders, exception management and stable policies. Integrated MRP–DRP planning links customer demand to distribution, production, procurement and supplier decisions. Digital and AI tools can strengthen this process, but they require governance, transparency and human accountability.

Key Terms

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

Independent demand: Demand originating from customers or external markets.

Material Requirements Planning (MRP): Time-phased calculation of materials and component replenishment.

Distribution Requirements Planning (DRP): Time-phased planning of replenishment across a distribution network.

Master Production Schedule (MPS): Schedule specifying quantities and timing of end-item production.

Bill of materials (BOM): Structured record of components and quantities required for a parent item.

Requirements explosion: Translation of parent requirements into lower-level component requirements.

Gross requirement: Total requirement before considering available inventory and scheduled receipts.

Net requirement: Requirement remaining after available supply is considered.

Projected available balance: Expected inventory balance after requirements and receipts.

Scheduled receipt: Supply already ordered or released and expected in a specified period.

Planned order receipt: Supply the system proposes to receive.

Planned order release: Order authorization scheduled after lead-time offsetting.

Lot-for-lot: Lot-sizing rule that orders exactly the net requirement.

Lead-time offsetting: Scheduling a release earlier than its required receipt by the lead time.

Capacity Requirements Planning (CRP): Translation of planned orders into resource loads.

Nervousness: Unnecessary instability in planned order quantities or dates.

Pegging: Tracing a requirement to the parent order or demand source that caused it.

Time fence: Boundary restricting or controlling schedule changes.

Safety stock: Inventory held to protect against uncertainty or service failure.

Review Questions

Short Answer Questions

1. Distinguish independent demand from dependent demand and give one example of each.
2. What information is normally contained in an MRP inventory record?
3. Explain the difference between a planned order receipt and a planned order release.
4. Why can an apparently correct MRP plan still be infeasible?
5. What is the purpose of a time fence?

Long Answer Questions

1. Explain the relationship among the sales and operations plan, MPS, BOM, MRP and CRP.
2. Compare lot-for-lot, fixed-order-quantity, periodic-order-quantity and EOQ approaches.
3. Explain how inaccurate BOMs, inventory records and lead times affect MRP performance.
4. Compare MRP and DRP in terms of purpose, inputs, outputs and network relationships.
5. Discuss the causes and managerial control of MRP nervousness.

Analytical / Application Questions

1. An item has 25 units available, gross requirements of 10, 30 and 20 units in successive weeks, and a scheduled receipt of 15 units in week 2. Using lot-for-lot and no safety stock, calculate planned receipts and ending balances.
2. A warehouse needs 150 units in week 9, has 45 units available, must retain 15 units of safety stock, and has a one-week transport lead time. Determine the planned receipt and release, assuming no other supply or demand.
3. A component supplier has a four-week lead time and a minimum order quantity of 200 units. Evaluate the operational and inventory consequences when MRP generates a net requirement of 130 units, and recommend a managerial response.

Multiple Choice Questions with Answers

Question 1

Which item is normally the primary output of an MPS?

A. End-item quantities by time period

B. Supplier invoice values

C. Warehouse picking routes

D. Quality inspection results

Correct Answer: A. End-item quantities by time period

Question 2

A parent planned order release of 80 units requires four units of a component per parent. What is the component gross requirement, assuming no scrap?

A. 20 units

B. 84 units

C. 320 units

D. 400 units

Correct Answer: C. 320 units

Question 3

Under lot-for-lot sizing, a net requirement of 145 units produces a planned order receipt of:

A. 100 units

B. 145 units

C. 150 units

D. 200 units

Correct Answer: B. 145 units

Question 4

If a planned receipt is required in week 10 and lead time is three weeks, the normal planned release is in:

A. Week 7

B. Week 8

C. Week 10

D. Week 13

Correct Answer: A. Week 7

Question 5

Which condition most directly indicates a capacity problem rather than a material-netting problem?

A. A BOM has an obsolete revision

B. Projected inventory is below zero

C. Required work-centre load exceeds available capacity

D. A scheduled receipt is recorded

Correct Answer: C. Required work-centre load exceeds available capacity

Question 6

What is the principal purpose of pegging in MRP?

A. To calculate EOQ

B. To trace a component requirement to its demand source

C. To measure warehouse utilization

D. To replace the BOM

Correct Answer: B. To trace a component requirement to its demand source

Question 7

DRP primarily plans:

A. Replenishment across distribution locations

B. Employee compensation

C. Product design tolerances

D. Annual financial depreciation

Correct Answer: A. Replenishment across distribution locations

Question 8

A warehouse has 60 units, demand is 40 units, and required safety stock is 10 units. With no scheduled receipt, its net requirement is:

A. 0 units

B. 10 units

C. 40 units

D. 50 units

Correct Answer: A. 0 units

Question 9

Which practice is most likely to reduce MRP nervousness?

A. Changing all planned orders daily

B. Removing lead times from records

C. Using time fences and firm planned orders

D. Ignoring supplier performance

Correct Answer: C. Using time fences and firm planned orders

Question 10

The classical EOQ model assumes, among other conditions:

A. Highly variable demand and unlimited shortages

B. Constant demand and known ordering and holding costs

C. No holding cost and finite replenishment uncertainty

D. A multi-level BOM with mandatory lot-for-lot sizing

Correct Answer: B. Constant demand and known ordering and holding costs