Open the item master for a part you make. Look at the planned lead time. Now find out where that number came from and who changed it last.
Why the closed loop everyone cites has never closed around the fields it describes, why five separate traditions derived the return path and none of them wired it, and what a supply chain leader should ask a vendor before buying an agent.
In most plants the answer is that it arrived in the initial data load, was adjusted once during hypercare because orders were finishing late, and has not been touched since. The routing standard values behind it were set by a work study department that no longer exists. Meanwhile the shop floor has reported every operation completion for years.
That is the gap. Not a missing algorithm. A field that nobody writes to.
Closed loop MRP has been the standard term since the 1970s. Planning, execution, feedback, corrective action. Every planning system on the market implements it, and implements it correctly.
Look at what it actually closes. Confirmations return quantities and statuses. Order status drives receipt quantity and reservation quantity. The next regeneration converts the difference into exception messages: past due, reschedule in, reschedule out, cancel. That loop works. It has worked for fifty years.
Now list what it does not touch. Planned lead time. Queue time. Lot size. Rounding value. Scrap and yield. Safety stock. Capacity assumption per work centre. Every one of those sits in the item master, the routing or the resource as a policy field. Every one of them is an input to the loop rather than an output of it.
Harrison and Petty put this in writing in 2002. Their reference model lists planning changes such as lead times or lot sizes among the disturbances entering the system. Not among the results leaving it. The architecture was explicit about it, and the point passed without argument, because the order loop was doing its job and the plans looked plausible.
This is the part that should end the argument about whether adaptive planning is technically possible.
The German PPS school published the derivation in the nineties and Schuh carried it into a standard handbook in 2006. The funnel model relates input, WIP and output at a work centre. Mean range equals WIP divided by mean output rate. Vary WIP across observed operating states and you get the production operating curve, from which you read the planned lead time that belongs to a chosen operating point. In a case reported by Wiendahl and Nyhuis, a coating work centre ran at 26 standard hours of WIP, deep in the overload region. The curve said 5 hours was reachable without notable output loss. It was implemented, and they report a throughput time reduction at that work centre of roughly 80 percent.
The same school did it for stock. The storage operating curve sizes safety stock from the measured maxima of three plan deviations: receipt date deviation, demand rate during the replenishment lead time, and quantity under delivery. In the article spectrum they work through, target stock decomposed into 43 percent lot stock, 37 percent demand rate variation, 20 percent quantity deviation and under 1 percent delivery date deviation. Those figures come from their worked example rather than from a survey, and the point is not the split itself. It is that a planner can be told which cause to attack, and in what order.
Wild did it from the inventory side in 1997. Safety stock equals the service factor times the measured deviation times the square root of the lead time. The MAD is smoothed every period. The lead time field is updated from measured actuals by exponentially weighted average, by supplier group. The forecast smoothing constant is reset by the tracking signal. He names the owner. He states the cadence: weekly or monthly, results inside three days of period end.
Herrmann's 2006 handbook carries a third version. SLIM-M at Samsung sets step level cycle time targets by allocating measured queue time to the constraint steps, then converts them into WIP targets through Little's Law.
Stadtler and Kilger, the canonical APS text, name the mechanism as a first class concept. Ex-post feedback is one of three coupling parameter types that have to be specified during model building, alongside instruction downward and reaction upward. The average realization of guidelines from the short term modules, they write, should be used for model adjustment in master planning.
Sethi and colleagues supplied the last piece in 2005, and it is the piece a business case needs: the marginal value of a forecast improvement in closed form, per stage, so the next unit of forecasting effort can be allocated on return rather than on conviction.