Every manufacturer running MRP has experienced the same frustration. The plan looks correct when it leaves the system. By lunchtime, it no longer reflects what is happening on the floor. Material that MRP said was available turns out to be on hold. A production order that MRP showed as on track has fallen behind. The schedule that seemed solid at 8am is being improvised around by 11am. This is not a failure of MRP. It is a failure of the data MRP runs on. --- What MRP Actually Does — and What It Assumes MRP calculates material requirements based on a snapshot of demand, inventory, and production status at a point in time. The calculation itself is correct. Given the inputs it receives, MRP produces the right answer. The problem is that the inputs are almost always behind reality. MRP assumes that the inventory positions in ERP are current. In most plants, they are 4–8 hours old. MRP assumes that production orders are progressing as planned. In reality, some are ahead, some are behind, and some have been informally adjusted by floor supervisors without updating ERP. MRP assumes that all available inventory is actually usable. In practice, some batches may be on quality hold — entered in the quality system but not yet propagated to ERP inventory status. MRP assumes that customer orders reflect actual committed demand. In reality, revisions sent via WhatsApp or informal email may not yet be in ERP. --- The Four Gaps Between MRP and Reality MRP Assumes Reality Often Is Cost of the Gap Inventory positions are current Positions are 4–8 hours old due to posting lag Late replenishment triggers; material shortages at staging Production running at standard yield Yield varies by batch, line, and shift Material imbalances that accumulate invisibly across shifts All available inventory can be used Some batches are on quality hold not yet in ERP Production scheduled against unavailable material Customer orders are as entered in ERP Informal revisions via WhatsApp not yet processed Wrong priorities; wrong quantities produced against incorrect demand Each gap individually creates operational disruption. All four operating simultaneously — which is typical in most mid-market plants — creates the daily pattern of improvisation and expediting that experienced operators have simply learned to expect. --- Why More Frequent MRP Runs Don't Fix the Problem The instinctive response to MRP inaccuracy is to run MRP more frequently. If the plan goes stale within hours, run MRP every hour. Some ERP implementations even offer continuous MRP — re-running the calculation after every transaction. This does not fix the problem. It accelerates it. More frequent MRP runs produce more frequent plan changes. Each plan change requires the floor to respond — adjusting work order sequences, restaging materials, communicating changes to operators. If the data feeding MRP is still hours behind reality, more frequent runs just create more churn from plans that are still wrong. The operators and supervisors who manage the floor learn to discount frequent plan changes. They develop their own informal view of what is actually needed — based on what they can see on the floor — and manage against that rather than against the ERP plan. The MRP becomes a compliance document rather than an operational tool. The fix is not more frequent MRP runs. The fix is better input data. Specifically, inventory positions and production statuses that update within minutes of floor events rather than at end of shift. --- The Posting Lag Problem in Detail Posting lag — the time between when a floor event occurs and when it appears in ERP — is the primary mechanism that separates MRP from reality. In most mid-market manufacturing plants, posting lag runs 4–8 hours for most event types. Production completions are entered at end of shift. Material consumption is posted when the operator has time. Quality holds are entered by the quality team when the investigation is complete, not when the hold is placed. During those 4–8 hours, every MRP calculation runs on a view of the world that is already wrong. The replenishment calculation fires late. The capacity check runs against a schedule that no longer reflects actual work order status. The material availability check misses batches on hold. The cumulative effect is a planning system that is nominally running in real time but is practically operating on a 4–8 hour delay. For a production environment where significant events happen multiple times per shift, this delay makes MRP directionally useful but operationally unreliable. --- What Closes the MRP-to-Reality Gap Three operational changes close the gap between MRP and floor reality. None of them requires a new ERP system or a new MRP module. Real-time inventory posting. When material is consumed, issued, or received, the ERP inventory position updates within minutes. Operator-facing interfaces designed for speed — a single confirmation tap rather than a full ERP transaction entry — make real-time posting practical without slowing the production floor. This single change addresses the most common MRP inaccuracy driver and typically reduces material shortage events by 30–40% within 60 days of implementation. Production confirmation feedback. When a work order completes or a yield deviation occurs, the production planning engine updates immediately. The next MRP calculation runs on what actually happened — not on what the standard said should happen. This feedback loop also feeds better standard cost data over time, improving the accuracy of future planning cycles. Quality hold propagation. When a quality technician places a batch on hold, that hold status is immediately reflected in ERP inventory. MRP never schedules production against a batch that cannot be used. The phantom availability problem — which creates more last-minute production stoppages than almost any other planning failure — disappears by system design. --- Measuring the Gap Before and After Before implementing real-time data flows, measure the current gap. Four metrics capture it: Average posting lag by event type: measure the time between a floor event occurring and its appearance in ERP for production completions, material consumption, quality holds, and goods receipts. This is the baseline that the improvement must move. MRP-vs-floor discrepancy rate: how often does the production floor discover a material or capacity situation that MRP did not predict? Track it weekly. This is the operational cost of the data gap. Emergency procurement frequency: how often does the procurement team buy material on an expedited basis? This metric falls directly as MRP accuracy improves and replenishment triggers fire on time. Schedule change frequency: how many times per shift is the production sequence informally changed because the plan turned out to be wrong? This falls as real-time data closes the gap between what MRP says and what the floor actually needs. After implementing real-time data flows through a manufacturing execution layer, these four metrics improve simultaneously — because they share the same root cause. The MRP is not wrong. The data it runs on was wrong. Fix the data, and MRP becomes the operational tool it was designed to be. --- The Compounding Effect of Multiple Gaps In isolation, each gap between MRP and reality creates manageable disruption. A single quality hold that is not in ERP for four hours is an inconvenience. A single batch of consumption data that is four hours late is a minor planning inaccuracy. When all four gaps operate simultaneously — as they do in most mid-market manufacturing plants — their effects compound rather than add. The quality hold creates phantom availability. The consumption lag means the inventory position MRP uses is already wrong. The informal priority change means MRP is scheduling against customer orders that no longer reflect actual priority. The stale lead time assumption means the replenishment triggered today will arrive later than MRP expects. The result is not four small errors. It is a planning system whose output requires significant manual interpretation before it can be used operationally. Experienced planners learn to adjust for the known biases — they know that MRP inventory positions are typically overstated by a certain amount, that quality holds are rarely captured in real time, that the afternoon demand signal is more reliable than the morning one. This adjustment is valuable expertise. It is also fragile: it exists in people's heads, not in the system. --- Measuring the Gap Before and After Before implementing real-time data flows, measure the current gap with four specific metrics. Average posting lag by event type: measure the time between a floor event occurring and its appearance in ERP for production completions, material consumption, quality holds, and goods receipts. This baseline reveals which event types are creating the largest gaps. MRP-versus-floor discrepancy rate: how often does the production floor discover a material or capacity situation that MRP did not predict? Track it weekly. This is the operational cost of the data gap expressed as a frequency. Emergency procurement frequency: how often does the procurement team buy material on an expedited basis per month? This falls directly as MRP accuracy improves and replenishment triggers fire on time. Schedule change frequency after plan release: how many times per shift is the production sequence informally changed because the plan turned out to be wrong? This metric falls as real-time data closes the gap between what MRP calculates and what the floor actually needs. After implementing real-time data flows through a manufacturing execution layer, all four metrics improve simultaneously — because they share the same root cause. Fix the data, and MRP becomes the operational tool it was designed to be.