Improving the management and execution of construction projects requires getting everyone on the same page. Two widely-used methodologies – the Critical Path Method, which emphasizes sequencing and interdependencies of tasks, and the Last Planner System®, which fosters collaboration and adaptability among jobsite teams – use different approaches to achieve this goal.
When teams don’t align, a gap forms between the big-picture master schedule used to map progress from beginning-to-end of the project and the production schedule used to manage the day-to-day, week-to-week progress. Such a gap leads to miscommunication, delays, and other costly problems as the actual work on the site drifts away from the expected outcomes in the high-level, master schedule.
In this article, we’ll explore the Critical Path Method (CPM), and the Last Planner System (LPS) to better understand their history and key concepts. We aim to highlight the importance of integrating these methods to bridge scheduling gaps and optimize construction workflows.
Levels of Schedules and Associated Methods in Construction Projects
Before we dive into the details of CPM and LPS, it is helpful to review different levels of scheduling done in large construction projects.
One of the most important tasks at the beginning of a construction project is when the General Contractor creates the baseline schedule (the long-term schedule). The baseline schedule represents the plan agreed upon by the Owner and the General Contractor for how the General Contractor will execute the project. This schedule is typically created using the CPM scheduling methodology, in which all the tasks needed to complete a project are defined and put in sequence. The baseline schedule is used for tracking progress but also for determining when payments will be made by the Owner to the General Contractor based on agreed upon milestones.
The master schedule represents the working version of that baseline schedule that includes the detail needed to manage construction at a high level. As construction proceeds, the master schedule evolves to reflect the evolution of the intended plan based on how the work is proceeding. When teams face large problems such as supply chain delays or unexpected events disrupting construction, the schedulers revise the baseline schedule and master schedule.
On the job sites, teams use production schedules at various levels of scope and detail to manage the work. Increasingly, this level of scheduling is created using The Last Planner System methodology. While the master schedule shows major milestones, the production schedule provides much more specific plans for how these tasks will be completed. This includes crew planning, material delivery, equipment usage, and many other project details. The Last Planner System seeks to engage the people doing the work to collaborate and optimize the workflows, achieving savings and efficiencies based on their hands-on understanding of the project.
Each method approaches planning from a different angle, using its own set of concepts and terms to plan and organize the construction process. As we will discuss below, a troublesome gap between the schedules forms when a two-way flow of information does not occur. We will now explain the perspective and key terms used for both methods so we can better understand how to bridge the gap.
What is the Critical Path Method (CPM) for Construction Project Management?
CPM scheduling has been a mainstay in construction project management for nearly 60 years. In the late 1950s, Morgan Walker of DuPont and James Kelley of Remington Rand developed the approach in response to a request: “See what you can do about scheduling.”
CPM emphasizes the interdependencies of tasks as these relate to scheduling, sequencing, and duration of the tasks, and the impact of these interdependencies on the entire project timeline. To ensure smooth progression throughout the project, all tasks must be clearly defined and manageable. CPM focuses on the logical scheduling connections among construction activities and on identifying the longest sequence of dependent critical activities that directly impact a project’s completion date – the critical path. The critical path determines the minimum project completion time.
CPM ensures tasks finish in a logical sequence. For example, the plumber and electrician complete their rough-ins before the drywall specialist installs and sands the drywall. Otherwise, changes to plumbing or electrical work could require cutting into drywall and repairing the cuts. Construction team leaders use CPM-based software, mostly Oracle Primavera P6 and MS Project, to create and manage the contractual project schedule. The master plan is a coarse rendering of the tasks in the schedule.
Here are some key terms to help understand CPM.
Activity Sequencing: CPM meticulously outlines all project activities and establishes logical relationships among those activities. CPM then determines which tasks must finish before other tasks can begin. This interdependence is often visually represented in a Gantt chart, a bar chart illustrating the start and end dates of each activity, and their relationships.
Float: Activities not on the critical path have float, representing the permissible delay without affecting the critical path. While float provides scheduling flexibility, mismanaging or failing to monitor float will lead to delays as float activities become critical path activities.
To effectively implement a CPM schedule, managers typically follow a series of steps:
- Create a work breakdown structure. Managers break the project down into smaller, manageable tasks.
- Develop a CPM diagram. Managers construct a visual representation of task dependencies.
- Forward pass calculation. Managers identify the earliest start and finish dates for each task.
- Backward pass calculation (float). Managers determine float by calculating the latest start and finish times.
With its focus on detailed activity breakdown and critical path analysis, CPM proves valuable for large-scale projects demanding precise scheduling and sequencing, and adherence to strict deadlines. However, CPM has limitations pertaining to task scheduling, particularly in its susceptibility to errors in initial estimates and its inability to inherently account for resource constraints.
What is the Last Planner System?
The term “Last Planner System” is the way that people who are using Lean Construction talk about the scheduling process. Lean Construction, inspired by the Toyota Production System, was coined in 1993, but proponents trace the origins to the construction of the Empire State Building starting in 1929. The Last Planner System, the umbrella planning term most often used, prioritizes waste minimization and continuous improvement throughout the construction process. As part of its holistic approach, the Last Planner System focuses on optimizing the entire project workflow and maximizing value delivery.
One of the main advantages of the Last Planner System is its collaborative nature. While CPM focuses on the scheduling interdependencies of tasks, the Last Planner System emphasizes the efficient coordination and cooperation of jobsite teams to minimize downtime, minimize waste, and enhance workflow reliability. This approach fosters a planning environment where commitments among trades are managed in short execution windows, allowing for continuous improvement and transparency.
The Last Planner System emphasizes empowering those directly responsible for executing the work to create and manage their schedules. This bottom-up approach to Lean construction planning fosters ownership and promotes realistic task durations, leading to greater schedule reliability. LPS often utilizes visual tools, such as sticky note walls, to track progress and identify bottlenecks.
A key performance indicator is the Percent Plan Complete (PPC), which measures how effectively teams are performing week-over-week at meeting their commitments. This metric provides the percentage of commitments teams are hitting on time, reinforcing the commitment to maximize both project and customer value.
Other key practices of the Last Planner System include:
Milestone Planning
The practice of conducting milestone planning by setting incremental goals that fit within the Master Schedule.
Pull Planning
A collaborative technique that begins with the project’s end goal and works backward, involving all stakeholders in determining the sequence of tasks and dependencies. By engaging those closest to the work in planning, pull planning fosters a shared understanding of goals and dependencies, promoting accountability and reducing rework.
Daily Huddle
A meeting at which the team reviews the previous day’s completed activities and discusses the current day’s tasks. This helps maintain safety, coordination, and adherence to plans, while fostering team culture and habits.
Look Ahead Planning
The practice of breaking down larger project sequences into actionable, short-term tasks, ensuring teams have clear and achievable goals within a specified timeframe.
Waste Removal
The focus on process improvement to minimize or eliminate eight forms of waste in construction: overproduction, waiting, unnecessary transportation, over-processing, defects, inventory, motion, and unused talent. Achieving this goal leads to cost savings, improved productivity, and enhanced customer satisfaction.
Collaborative Learning
The promotion of collective accountability and systematic learning to achieve better project outcomes. Teams work together to identify constraints, share knowledge, and collectively solve problems to improve workflow reliability. Through Plan-Do-Check-Act (PDCA) cycles, teams analyze performance data, identify deviations, and implement better practices.
The Last Planner System does not attempt to predict the flow of work over a long period the same way that CPM does. Instead the focus is on a much shorter time scale. The Last Planner System unlocks the intelligence and creativity of the teams on the ground, addressing directly the idea that schedules must change and adapt as work is performed and problems arise and are solved.
Both of these methods are crucial to the success of large construction projects. Communication and execution goes much better when all teams understand what is happening in both levels.
Looking for a better way to bridge the gap between CPM and LPS?
Touchplan® is the leading real-time construction planning platform that helps teams:
- Improve collaboration between schedulers and field teams
- Enhance workflow reliability by aligning high-level planning with daily execution
- Reduce delays and rework with real-time progress tracking and insights
Learn how Touchplan® can help your team unite CPM and the Last Planner System. Schedule a demo today.