## 2 Literature Review
### Multiskilled Resources Scheduling (MSRS)  
Resource shortage and low productivity are two of the major challenges facing the construction industry [11; 22]. Multiskilling is a resource utilization approach that has been identified as a promising strategy for addressing these two challenges [11; 22; 23]. A multiskilled resource is simply a worker who has more than one trade skill and competence to carry out multiple tasks of a project i.e., the ability of _a plumber_ to carry out _electrical work_[11]. Many studies in the past have acknowledged the benefits of multiskilled resource strategy [23]. These benefits include but are not limited to an increment in earnings of the multiskilled workers, a 35% reduction in total labor requirement, 5-20% labor cost savings, and a 47% increase in average employment duration. Recently, [PERSON] et al. [22] stipulated that improved safety onsite, improved project quality, flexible staff deployment, easing the shortfalls of skilled labor, improved job satisfaction, and improved competence of workers as additional benefits of implementing multiskilled resource strategy in the construction industry. Moreover, in the last five decades, these benefits had led to the continuous evolution of MSRS best practices through the development of various mathematical and heuristic scheduling models/algorithms by researchers and practitioners. [PERSON] et al. [23] developed an analytical model that was used to assess the impact of four multiskilling strategies on a construction project worth USD 70,000,000. The study noted that the multiskilling strategy could yield up to 20% labor cost savings among other benefits. In like manner, [PERSON] et al. [24] proposed a linear programming model for optimizing a multiskilled workforce thereby realizing up to 20% improvement in workforce performance. However, the authors stated that the application of the model is limited to the partial employment situation. Furthermore, [PERSON] et al. [11] introduced a heuristic multiskilled algorithm for scheduling resource-constrained projects using resource substitution rules. The algorithm utilizes resource information to resolving resource conflicts by substituting the under-allocated resources for the over-allocated ones based on predefined resource substitution rules and relative to the available multiskilled resources in the resource pool. In the same way, [PERSON] and [PERSON] [25] presented an augmented algorithm for multiskilled resource scheduling based on the resource substitution approach and resource-driven activity duration. The author adopted vertical resource assignment to generate optimal and feasible resource schedules. In another study, [PERSON] and [PERSON] [26] introduced a constraint programming (CP-based) optimization model for generating an optimal schedule in linear projects involving complicated combinatorial scheduling problem based on the integration of single/multiskilled crews. Further, [PERSON] and [PERSON] [27] developed a hybrid-benders decomposition (HBD) algorithm using mixed-integer linear programming (MILP) and constraint programming (CP). In the study, both MILP and CP's strengths were combined to resolve the resource conflict of an assignment type of RCPSP leading to a minimized total staffing cost of multi-skilled personnel. [PERSON] [28] examined a multiskilled project portfolio scheduling problem (MSPPSP) using a metaheuristic algorithm based on differential evolution (DE). The result revealed a minimized project duration deviation and efficient resource assignment for each project of the project portfolio. Priority Rule (PR)  
In MSRS, PR plays a significant role in the development of a feasible resource schedule. PR is a technique used in making decisions when dealing with a generic RCPSP. It is used to determine the order in which resources will be allocated to each activity during scheduling. Table 1 shows some of the popularly used PRs in MSRS. In their research, [PERSON] et al.