## 3 Conceptual Model: Procurement Order Quantities Regulation for Shortage Scenarios
### Factors in Relation to the Procurement Order Quantity, Service and Cost Levels  
These factors determine the speed and effectiveness with which external changes to the system, such as changes in demand, are absorbed and attenuated by the planning model so that effectiveness is not compromised and efficiency, e.g., cost or service oriented, that is, quality and time, in that situation is maximized. Critical factors for supply management for a manufacturing company are:  
* Expected demand for the final product;
* Current inventory/stock level;
* Supplier replenishment time;
* Supplier procurement lot size. These factors may suffer from the following deviations that are a source of uncertainty for planning:  
* Deviations in the expected demand for the final product (forecast error and/or lack of firm customer orders);
* Inventory deviations;
* Deviations in the procurement order lead time (supplier delivery date);
* Deviations in the procurement order quantity (quantity delivered from supplier). These deviations are compensated with stock-outs and supply failures that trigger production losses of available capacity or the need to make changes in planning and therefore increase the needs of other procurement goods. To avoid these effects, there are two mechanisms to compensate for these potential negative effects that nevertheless have a cost (time, space, material):  
* Safety stock;
* Security times. In these regard, Figure 4 shows how the the procurement order quantity is determined depending on the planning type and on the need of compensation factors:  
These factors will then be the determining factors to analyze the model's responses in the simulation; for example, an already placed procurement order that changes from level 1 to level 2 in the form of a step change due to a procurement shortage and how the system responds to different procurement order quantity configurations. The inventory function can be described from two different perspectives. From a classical planning perspective, a high level of stock allows for trouble-free production, short delivery times, overcoming production failures, constant production utilization, and thus economical production. However, the idea of efficient management is based on the view that stocks are generally used only to hide processes deviations and failure, uncoordinated capacity, lack of flexibility, waste, and lack of reliability and delivery. Flexibility and adaptability can be understood at different levels--stock transfers, supply synergies, capacity coordination, and production networks--thanks to an MRP approach. A factory is intended to be classified at different levels. Optimization, to improve efficiency, must be performed at all levels. The lower level is the process level, where losses are due to  
Figure 3: Parameter system target (own elaboration). the physical movement of the machine or of materials or people in relation to the process itself. Moreover, at the production or transport machine level; the line level that contains many machines and the factory level must also be considered and optimized to provide a comprehensive result [42]. 4 Methodological Simulation Approach Depending on Supplier's Behavior and Procurement Order Quantity  
According to the \"VDI-Richtlinie\", simulation is the reproduction of a system with its dynamic processes in an experimental model capable of acquiring knowledge that can be transferred to reality. In particular, processes which develop over time [43]. In contrast to analytical models that offer the possibility of reaching an optimal solution, the main problem with simulation models is that these models do not provide a closed set of solutions [44]. Simulation models are used mainly to support decision making as they reveal a system's dynamic behavior [45]. On the other hand, simulations are the only practical way to test models, because our mental models are dynamically deficient, omitting evaluations, lags, accumulations, and nonlinearities [46].