Synchronous unpaced flow lines with worker differences and overtime cost

In this paper, we consider the design of a synchronous, unpaced flow line where workers operate at different skill levels and overtime is used, if necessary, to meet a daily production quota. The line is unpaced in the sense that items only move to the next workstation when all workers on the line have completed their respective tasks. The design problem in this case is to assign both workers and tasks to workstations to minimize the expected sum of regular and overtime costs. To solve this problem, we develop an optimization algorithm for smaller problems and a heuristic algorithm for larger problems, which we use to investigate the sensitivity of total expected cost to changes in the price of overtime, hiring practices, worker differences, and the overall amount of work time variability. Based on an extensive computational analysis, we found that (1) planned overtime is frequently beneficial, (2) more workers should be hired as worker variability increases, and (3) increases in overtime costs frequently yield a relatively lower percentage increase in total expected cost. Other managerial implications are discussed in the paper.