The growing need of responsiveness for manufacturing companies facing market volatility raises a strong demand for flexibility in their organisation. Since the company personnel are increasingly considered as the core of the organisational structures, a strong and forward-looking management of human resources and skills is crucial to performance in many industries. These organisations must develop strategies for the short, medium and long terms, in order to preserve and develop skills. Responding to this importance, this work presents an original model, looking at the line-up of multi-period project, considering the problem of staff allocation with two degrees of flexibility. The first results from the annualising of working time, and relies on policies of changing schedules, individually as well as collectively. The second degree of flexibility is the versatility of the operators, which induces a dynamic view of their skills and the need to predict changes in individual performance as a result of successive assignments. We are firmly in a context where the expected durations of activities are no longer predefined, but result from the performance of the operators selected for their execution. We present a mathematical model of this problem, which is solved by a genetic algorithm. An illustrative example is presented and analysed, and, the robustness of the solving approach is investigated using a sample of 400 projects with different characteristics. . The case where the two workforce productivities, the 'worker-based' and the 'skill-based', one is homogeneous and the other is heterogeneous, we called it 'mixed performance'. One can find this case in the hierarchical classification of workforce productivities, e.g. senior, standard and junior classification that was adopted by Yoshimura et al. (2006), Valls, Perez, and Quintanilla (2009. The workforce productivities were considered as heterogeneous in the case where neither the worker-based productivity, nor the skill-based productivity is homogeneous. According to the best of our knowledge, the first description of this heterogeneous characteristic is the work of Nelson (1970) in a dual resource constrained job shop. This heterogeneous nature was then adopted in many applications, e.g. the works of Attia, Edi, and Duquenne (2012), Kolisch and Heimerl (2012) or Yannibelli and Amandi (2012). Here also, the heterogeneous nature will be considered in addition to the evolutions of productivity as a function of previous workforce allocation (i.e. the dynamic aspect of productivity).Additionally, the workforce flexibility also relies on a temporal concern that plays a vital role in this subject. This flexibility can be developed along two main axes: the traditional overtime working hours and the working time modulation under annualised hours (AH). AH strategy is the possibility to spread irregularly a number of working hours that is predefined on a specified period (often one year), provided some constraints are respected. According to Corominas, Lusa, and Pastor (200...
