The Critical Path Method, or CPM, and the Last Planner System, or LPS, are not rival methodologies that solve the same problem, but two different layers of the same planning system: one models the project in time and supports its contractual framework; the other controls production reliability on site. Presenting them as mutually exclusive confuses the strategic level with the tactical level.

Each approach responds to a different objective and acts on a different time horizon.

The Critical Path Method (CPM)

CPM is a scheduling technique based on networks of activities with logical precedence relationships. Through forward and backward passes, it calculates the early and late dates of each activity, float and the critical path, the sequence with zero total float that determines project duration. It is a deterministic model: given logic and durations, it produces a completion date.

Its functions are fundamentally strategic and contractual:

  • Establishing the programme framework and auditable contractual baseline.
  • Monitoring milestones and managing interfaces at a high level.
  • Supporting loading and resource analysis at a global scale.
  • Supporting claims and extension of time requests, as well as time impact analysis and forensic analysis.

It is implemented using scheduling tools such as Primavera P6 or Microsoft Project. Its role is to structure the project in the long term and act as an objective contractual reference.

The Last Planner System (LPS)

LPS is a production planning system originating in Lean Construction. Its objective is not to calculate dates, but to increase workflow reliability through collaborative planning and short-term commitment control. It operates on the principle of shielding production: only constraint-free work is released to execution.

Its logic is articulated through the should-can-will-did cascade: what should be done, from CPM; what can be done once constraints have been removed; what the team commits to doing; and what was finally done. Its components are:

  • Phase planning or pull planning. Collaborative workshops that break down the CPM into executable sequences, planned backwards from each milestone with the participation of those who execute the work. It relies on physical or digital boards with cards.
  • Look-ahead planning. A three-to-six-week horizon dedicated to analysing and removing constraints, or make-ready: materials, information, permits, precedences and resources. Its function is to ensure that work reaches the execution week released.
  • Weekly work plan. Work is not assigned, it is agreed: teams commit only to tasks whose constraints have been resolved and which they know they can execute.
  • Daily huddles. Real-time adjustment of progress and reassignment in response to unforeseen events.
  • Reliability measurement, PPC or Percent Plan Complete. Percentage of tasks completed compared with those committed. PPC, together with analysis of reasons for non-completion, forms the system's continuous improvement loop.

Why both are necessary

The complementarity is structural, not a matter of preference. CPM is a push system based on deterministic logic and target dates; LPS is a pull system based on commitment and the actual availability of released work. In control terms, CPM defines the setpoint, the plan and contractual objectives, and LPS acts as the feedback loop that governs execution and corrects deviation.

  • CPM provides the time and contractual model: milestones, compliance, interface management and basis for claims.
  • LPS provides the production control layer: it converts that framework into a coordinated, reliable flow aligned with site reality.
  • PPC and its reasons for non-completion feed back into CPM: systematic deviations observed in production must be transferred to the update of the programme framework, closing the loop between tactics and strategy.

Using CPM as the only source for daily management is as ineffective as managing a high-value contract without an audited baseline schedule.

Practical implementation

The integration of both levels follows a defined sequence:

  • Develop the CPM schedule as the contractual framework, with milestones, detailed logic and loaded resources.
  • Convene phase planning workshops, or pull planning, to translate the CPM into executable sequences for the teams.
  • Establish Lean routines, look-ahead with constraint analysis, weekly plan and daily meetings, supported by collaborative software such as Touchplan, VisiLean or Plannerly.
  • Appoint facilitators to ensure the connection between central planning and the site.
  • Measure PPC weekly and analyse reasons for non-completion to generate improvement actions.
  • Update the CPM periodically with the deviations recorded during execution.

The PlanBuilt approach

PlanBuilt applies this dual model as a working standard: CPM is maintained and controlled in Primavera P6 as the contractual baseline, while the production layer, weekly plan, commitment tracking and PPC measurement according to Last Planner methodology, is managed in an environment specifically designed for it and integrated into an interactive online system. In this way, reliability measured on site and the status of the programme framework coexist in a single data source, and the PPC-to-CPM update loop no longer depends on scattered spreadsheets.

Conclusion: more engineering, less ideology

It is not about choosing between CPM and LPS, but about assigning each one its function within the planning system of a complex project. Effective planning is not a methodological debate: it is the ability to anticipate, coordinate and execute reliably. CPM governs the strategy and contractual framework; LPS governs production. Their conscious integration, with PPC as the feedback mechanism between both, is what allows planning with foresight, control and adaptability.

To implement this dual system in a project, PlanBuilt offers both CPM control in P6 and the Lean layer for weekly plan tracking and PPC.