There are some critical differences between how MAS 500's MPR (material requirements planning) and IR (inventory replenishment) function. While both processes use the same underlying data and share many of the same algorithms, there are critical differences that users should understand.

One critical difference is that IR works from a "snapshot," whereas, MRP uses a time-phased approach to the analysis. IR takes a look at your current situation in order to try to determine what replenishment action should be taken at this time. MRP, on the other hand, places calculated data into a series of time-phased planning "buckets." The controlling factors for these MRP planning "buckets" are found (primarily) in the "Initialization Criteria" on the Options tab of the Material Requirements Planning Generation screen.


The range of dates for which an MRP version will plan is governed by the "Start Date" and the "Cut-off Date", but the size of each planning "bucket" is determined by the "Days in Planning Period" parameter. (Exceptions to this may be set on the MRP Options tab in Maintain Routings. These exceptions will force specificed Routings to be planned in different sized "buckets.")

If you see differences between IR "Suggested Order" quantities and MRP "Planned Orders" or other appropriate categories of replenishment, here's why:

  • For Items with a Reorder Method of "Min/Max," IR will not include Projected Demand in its calculations for "Suggested Orders". The reason for this is that IR, unlike MRP with its "planning buckets" by date, doesn't look into the future. IR assumes that "Min/Max" configuration is designed to cover your Projected Demand. MRP, on the other hand, will place Projected Demand into the appropriate time-phased "buckets" and include it in its calculations.
  • Also for Items with a Reorder Method of "Min/Max", IR will not included Safety Stock in its calculations. IR assumes that any "Minimum" stock level already includes any Safety Stock Requirements. On the other hand, MRP will calculate "Minimum" as minimum over Safety Stock quantities if there is a quantity in the Safety Stock parameter for the Inventory record.
  • One other factor that should be considered: IR does not do anything with Routings. That is to say, IR does not "blow down" through Routings to determine lower-level requirements or generate "Planned" Work Orders. Therefore, if an Item is component on a Routing and is required in the production of some other higher-level product, IR will not see the demand coming from a "Planned Work Order." MRP, on the other hand, will present these data as "Planned Material" entries.

I hope this helps clarify some of differences between MAS 500's MRP and IR calculations. You should give careful consideration as to whether you need Inventory Replenishment or MRP to meet your specific requirements.

-- Richard D. Cushing