MRP common mistakes
MRP · Understand the traps and how to avoid them.
Treating the MPS as identical to the sales forecast and using forecast quantities directly as scheduled receipts in the MRP grid.
Candidates conflate demand management (forecast, customer orders, ATP) with master scheduling, so they forget the MPS is a statement of what will be built, validated against capacity and buffer/backlog policy, not a restatement of demand.
What to do instead
For every MPS question, first identify whether the input is forecast, actual orders, or the agreed build plan. State that the MPS is a time-phased build schedule for end items or planning bills, derived from but not equal to demand, and that forecast consumption and ATP govern how orders are netted against it.
Exploding a multi-level BOM by multiplying quantities straight down the structure while ignoring the parent item's own lead time offset and the level at which the component is planned.
Candidates memorize 'multiply quantities' but forget that low-level codes determine when an item is exploded and that each parent's lead time shifts the need date of its components.
What to do instead
Work the explosion as a time-phased process: assign low-level codes, explode level by level, and offset each component's gross requirement by the parent's lead time. Practice on a three-level BOM with different lead times to see how need dates cascade.
Confusing gross requirements with net requirements, or netting against on-hand only while ignoring scheduled receipts and safety stock.
The netting formula has several terms and candidates drop safety stock or scheduled receipts under time pressure, producing an incorrect planned order quantity.
What to do instead
Write the netting equation explicitly each time: net requirement = gross requirement + safety stock − projected on-hand − scheduled receipts. Then apply the lot-sizing rule to the net requirement, not the gross, and carry the resulting projected balance forward period by period.
Applying the wrong lot-sizing technique or miscomputing setup versus holding cost trade-offs, especially for lot-for-lot, fixed order quantity, EOQ, and period order quantity.
Candidates know the names but not the assumptions; they use EOQ where lot-for-lot is specified, or they forget that POQ converts EOQ into a number of periods.
What to do instead
Build a comparison table of techniques with their inputs and assumptions. For calculation items, show the setup cost, holding cost rate, and demand pattern before choosing a method, and verify POQ by dividing EOQ by average period demand and rounding to whole periods.
Reading action messages as errors to be cleared rather than as planning signals, and failing to distinguish reschedule-in, reschedule-out, release, and cancel messages.
Candidates treat exception messages as system faults and pick 'delete the message' answers instead of interpreting what the planner should do to the order.
What to do instead
Map each message type to its cause and correct planner response: release when the order date has arrived, reschedule-in when need date moved earlier, reschedule-out when it moved later, cancel when demand disappeared. Always ask what changed in the plan to generate the message.
Treating MRP as a capacity plan and ignoring the role of RCCP and CRP in validating the material plan.
The hierarchy is taught as a list, and candidates collapse MRP and capacity planning into one step, missing that RCCP tests the MPS at a rough level while CRP tests the detailed MRP at work-center level.
What to do instead
Draw the hierarchy with inputs and outputs at each level: S&OP to MPS to MRP to CRP, with RCCP validating the MPS and CRP validating MRP. For any scenario, identify which level the question is testing before selecting a capacity technique.
Assuming infinite capacity or ignoring load profiles when interpreting CRP output, and confusing capacity requirements with available capacity.
Candidates compute load but forget to compare it to demonstrated or available capacity, or they overlook that CRP assumes infinite capacity and only highlights overloads.
What to do instead
For CRP items, always compute load per work center per period and compare against available hours, then state the resulting overload or underload. Remember CRP does not schedule; it identifies where capacity must be adjusted or the plan leveled.
Mismanaging item master and inventory record data: wrong lead time type, incorrect safety stock, wrong planner code, or ignoring shrinkage and scrap percentages.
These fields look like administrative detail, but each drives planning output; candidates skip them and then cannot explain why planned orders appear at the wrong date or quantity.
What to do instead
Review the item master field by field and state how each affects planning: lead time offsets need dates, safety stock raises net requirements, scrap inflates order quantities, and planner codes route action messages. Include scrap in the order quantity calculation when specified.
Confusing MRP II, ERP, and closed-loop MRP, or describing ERP integration as a simple data feed without addressing a single database, financial integration, and transaction discipline.
Candidates learn acronyms without the historical and functional distinctions, so they mislabel systems and miss why integrated data integrity matters for MRP accuracy.
What to do instead
Define each term precisely: closed-loop MRP adds capacity and execution feedback, MRP II adds financial and simulation capability, ERP extends across the enterprise on one database. For integration questions, emphasize single-source item and BOM data and the effect of transaction accuracy on plan validity.
Treating performance metrics as generic KPIs and not linking them to MRP-specific measures such as schedule attainment, plan stability, forecast accuracy, inventory turns, and data integrity.
Candidates default to broad business metrics and cannot connect a metric to the planning process that produces it or to the corrective action it implies.
What to do instead
For each metric, state the formula, the planning process it measures, and the typical corrective action. Distinguish delivery performance from plan adherence and inventory accuracy from inventory turns, and explain how poor data integrity degrades every other metric.