How the Master Production Schedule Changed Through Time
Executive Summary
- The term master production schedule changed through time and differs in critical ways from the term MRP.
- Four dimensions make up the MPS, but there are questions about whether an MPS is still necessary.
A Strangely Named Term
The Master Production Schedule or Master Production Schedule was always a problematic term. Firstly it never restricted its area of control to just production. Secondly, the term master seems to imply it is all-encompassing, when in fact, it was meant to mean the highest level type of analysis (only critical materials, no BOM explosion). However, what is interesting is that the term had one meaning back from the 1960s to the 1980s and changed.
The early definition of the Master Production Schedule sounds something like S&OP. The current usage of the Master Production Schedule is more tactical, and I will argue in this paper that there is a reasonable probability that the term how outlived its usefulness and is now confusing people more than describing actual events in systems.
The Change of Master Production Schedule Through Time
To prove the point regarding the alteration of the meaning of the Master Production Schedule, I will use three definitions of the Master Production Schedule to show that different authors seem to highly various aspects of what makes MPS different from MRP.
Wikipedia has the following definition of a Master Production Schedule. Surprisingly, this definition is missing an essential element.
“Due to software limitations, but especially the intense work required by the “master production schedulers”, schedules do not include every aspect of production, but only key elements that have proven their control effectivity (emphasis added), such as forecast demand, production costs, inventory costs, lead time, working hours, capacity, inventory levels, available storage, and parts supply. The choice of what to model varies among companies and factories. The MPS is a statement of what the company expects to produce and purchase (i.e. quantity to be produced, staffing levels, dates, available to promise, projected balance).” – Wikipedia
This is in line with the classical definition of the Master Production Schedule. Here is another quote in line with the classical description, which is from 1967 to 1985 (it is hard to know precisely when the quote originated as I do not have the first edition of the book from which I am quoting)
The Classical Definition of the MPS
“The master production schedule provides the mechanism by which policies are converted to plans, me process of translating policies to numbers (called master scheduling) requires teamwork among all functions. The set of numbers developed by this process is used to drive the formal system. Once policy has been established. the manufacturing planning and control manager develops plans to suit these. When the master production schedule is deemed realistic (achievable), the system goes to work. Plans for material and capacity requirements are generated in detail or updated, feedback during execution detests significant deviations from the plan and corrective actions are initiated.” –George Plossl
George Plossl discusses the Master Production Schedule as if it were more like an S&OP process. He considers it the first step in the overall planning process. He states that:
“The fundamental issue is really what types of bill of materials to use to convert the Master Production Schedule into the priority and capacity planning details view. One or more of the three different types of BOMs may be used:”
“Specific finished products
Major subassemblies
Planning modules”
“The greater the difficulty in forecasting the product model or combination of options in a product model that a customer will buy, the greater the need for a well-defined planning module at the BOM.” – George Plossl
However, while this may have been true when this was written, modern systems can quickly ultimately explode the BOM. There is now little need to discuss the BOM as being blown in single-level or multi-level, as it is easy for modern computers to explode the multi-level BOM every time.
What is the Master in Master Production Schedule?
The “master” in Master Production Schedule describes the most important or central products that the company plans. A Master Production Schedule does not include all products in the company’s product database. This is demonstrated by definition below.
“Main Idea: The Master Production Schedule is a form of MRP that concentrates planning on the parts or products that have the great influence on company profits or which dominate the entire production process by taking critical resources. These items are marked as ‘A’ parts (MPS items) and are planned with extra attention. These items are selected for a separate MPS run that takes place before the MRP run. The MPS run is conducted without a BOM explosion so that the MRP controller can ensure that the Master schedule items (MSI) are correctly planned before the detailed MRP run takes place.” – SAPIMG.com
For instance, when a company moves towards constraint-based planning, it does not set constraints for non-critical, non-Master Production Schedule products. This would be too much work. However, compared to how well-known MRP guru George Plossl uses the Master Production Schedule, it is hard to see if many companies even have a Master Production Schedule process.
Master Production Schedule Explicitly Stated
A Master Production Schedule run is four things that relate to the product, timing, constraining, and the BOM. These are listed below:
- Product: A subset of the overall product database
- Timing: Run ahead of the MRP run
- Constrained: MPS runs are restricted, which of course, is not possible with MRP (which is strange then that MPS is a type of MRP run.)
- BOM: The Bill of Material is not exploded with the Master Production Schedule
To keep these straight in people’s minds, I created the following graphic.
However, this is not at all how the term MPS is used today. In most cases, it is used interchangeably with the name MRP. This is replicated at least at one vendor. To find out how SAP uses the term MPS, see this article.
Master Production Schedule and Computing Power Restrictions
One also should question why an MPS run (and I speak of its classical meaning here) is required in the first place. Why not merely perform a regular MRP run with full BOM explosion and glean the necessary information for the MPS from this run. The answer is that this was not possible in the past due to the limited processing of computers when the MPS concept was initially developed. However, with today’s computers, it makes little sense to invest resources in developing a particular run used for MPS purposes. Therefore, in my view, the term MPS has outlived its usefulness as it is a method of restricting the load onto hardware resources that is from a different era.
To learn about the history of MRP, see this article.
The Four Factors that Make Up the Master Schedule for the Supply Plan
Different companies use the term Master Schedule or Master Production Schedule (MPS) is often inaccurate ways. In the vast majority of cases, it is not the official definition of the term. In many cases, it is used interchangeably with the name MRP or even the forecast, which is incorrect. It turns out that Wikipedia has a reasonably good definition of the MPS.
“A master production schedule (MPS) is a plan for individual commodities to be produced in each time period such as production, staffing, inventory, etc.[1] It is usually linked to manufacturing where the plan indicates when and how much of each product will be demanded.[2] This plan quantifies significant processes, parts, and other resources in order to optimize production, to identify bottlenecks, and to anticipate needs and completed goods. Since an MPS drives much factory activity, its accuracy and viability dramatically affect profitability. Typical MPSs are created by software with user tweaking.”
However, Wikipedia falls a bit short in distinguishing the MPS and other plans.
An MPS runs a combination of four things related to the product, timing, constraining, and BOM. These are listed below:
- Product: A subset of the overall product database.
- Timing: Run ahead of the initial supply planning run.
- Constrained: MPS runs are restricted, which of course, is not possible with MRP (which is strange then that MPS is a type of MRP run.)
- BOM: The Bill of Material is not exploded with the master schedule.
Why These Four Factors?
These factors make up the master schedule because it allows the master schedule to be created as almost a simulation run. The reason for each element is included below:
A Subset of the Overall Database
The reason for making the MPS run a subset of the overall product database is twofold.
- The first reason is that the MPS run to only be for critical parts.
- A second reason, which is now dated, is that earlier computer system was limited in processing capabilities. And order to get the MPS to run on time and possibly run it multiple times, it was essential to limit the amount of data the system had to process.
This leads to a related topic of whether MPS itself is an anachronism covered in this article.
Why Constraints?
This makes the master schedule more realistic. However, this is more of an additive factor. The master schedule runs tend to be constrained, but of course, a limited planning run could lack any of the other three elements and not be a master schedule.
Why Timing?
The MPS must be run before the network or initial planning run.
A simulation run like the MPS would make little sense if it were run after or at the same time as the initial planning run.
Why Limited BOM Explosion?
A quote well describes this from SAP.
“In the MPS menu there is a separate single-level planning run, which can be executed as single-item planning or total planning. This planning run only includes the master schedule items. Dependent requirements are created for the BOM level directly below the planning level. Levels below this, however, are not planned. This means that the MRP controller can authorize any changes to the master plan before they affect the various BOM levels.” – SAP Help
Interestingly, the sentence in blue is no longer a selling point of MPS, as it assumes that the system does not have a simulation environment. That is a copy of the system model that connects to the user interface). However, with modern systems, simulation environments can be used to perform planning at any detail level without affecting the “production” instance.
For more on MPS, see this article.
References
https://en.wikipedia.org/wiki/Master_production_schedule
“Production and Inventory Control: Techniques and Principles 2nd Edition,” George Plossl, Prentice Hall, 1985
“Production and Inventory Control: Applications,” George Plossl, George Plossl Education Services, 1983
https://www.sap-img.com/production/difference-between-mrp-vs-mps.htm