4. Methodology
4.1. Optimization model Bordoloi and Matsuo [7] addressed a linear program to model an assembly line for which the operations in its front-end stage, called stage A, should be completed prior to operations in the back-end stage, called stage B. Workers in the first knowledge level (new-hired) and in the second knowledge level (semi-experienced) perform the operations in stage A and B, respectively.Workers in the third knowledge level (fully experienced) are assigned to the production stages for training lower-level workers (i.e., new-hired and semi-experienced workers). They can also participate in the production process if they have free time. Fig. 1 illustrates both production stages and the possible assignment of workers at different knowledge
levels for doing operations in each stage. Moreover, all workers in the first two levels should be trained during the production process in order to move forward to a higher level at the end of production period. It should be noted that the only possibility here is to recruit people in the lowest level and workers in the other two levels cannot be directly hired. In other words, to meet stochastic demand, the production level can be controlled only by determining the number of new-hired workers for each period. They should therefore be trained during the production process in a specified time framework. This model has some assumption as follows:
1. No worker is allowed to work in both stages A and B in a specific period.
2. All predefined values of parameters remain unchanged within the planning horizon.
3. The probability distribution of demand is already known.