## 2 Current Approaches to Optimize the Efficiency of Construction Projects  
### Last Planner System  
The Lean Construction philosophy allows the management of construction projects through the reduction of cycle times and the simplification and minimization of the necessary processes to undertake an activity. This is achieved by increasing production flexibility and transparency throughout the process, through tools that allow global control of the process, reducing variability and establishing continuous improvement. One of the most representative tools of the Lean Construction philosophy is the Last Planner System. This tool allows an agile and proactive management during construction work, moving away from some traditional project management methods, such as the critical path method that only allows a late detection of deviations.  
In addition, these traditional methods are generally unrealistic with respect to the reality of what is actually executed, being simple expectations that are corrected during the execution of the work, simply adjusting to the deviations [24], and in many cases re-planning is abandoned. This because interconnections between environmental variables and factors that may influence project development are not linear relationships as traditional methods suggest, but are in fact complex relationships of causality and effect [25]. Given this gap, the Last Planner System offers the possibility of generating a realistic workflow based on a collaborative planning which is developed by all the agents involved, where the acquisition of commitments and the agility of replanning allows to face changes and unforeseen events that may arise during the development of construction work (Figure 2).  
All this confirms the capacity and suitability of the Last Planner System for the management of construction projects, and its variable and complex environment, since it provides versatility and agility. This agile management ensures better performance by minimizing variability, while providing reliability to work flow through commitments and mature decision making. This is achieved through the different phases of the Last Planner System that extend in different time horizons throughout the construction project: Master schedule, Phase Scheduling or Pull schedule, Look Ahead Planning and Weekly Work Plan. The system begins with an initial meeting where milestones and deadlines from the Master Schedule are established [26]. This phase known as Phase Scheduling or Pull Schedule as a result of the use of the Pull system to carry out a collaborative schedule of activities among all stakeholders, starting from the milestones or completion date from the end to the beginning, that is, in reverse to the date of termination [27]. This Phase Scheduling precedes the Look Ahead Planning phase, that links long-term planning with short-term commitments [28], at this point workflows of each activity in that period are identified, usually from 4 to 6 weeks in advance. Then, an analysis of the restrictions that may affect both the start and the development of planned work is performed, classifying them and establishing a responsible person and a date for their solution before the start of work. Then, restriction-free activities go to the next phase, the weekly schedule known as Weekly Work Plan, which is the most detailed planning before the start of work. Finally, performance is assessed through performance indicators called Percent Plan Complete (hereinafter PPC) which allows comparing planned activities with those executed.