Critical Path Method

A technique to determine the most efficient sequence of activities to complete a project.

Also called: Critical Path Analysis, Critical Path Network Analysis, Critical Path Scheduling, Critical Path Scheduling Method, and Critical Path Technique

See also: Cycle Time, Gantt Chart

Relevant metrics: Project Completion Time, Project Cost, Resource Utilization, Project Quality, and Project Risk Management

In this article

What is Critical Path Method?

The Critical Path Method (CPM) is a project management technique typically used to plan and control large-scale projects. It involves identifying the sequence of activities that must be completed in order to achieve a desired outcome, known as the critical path, which determines the shortest possible duration for the entire project.

The critical path includes tasks that directly affect the project’s completion date.

If any task on the critical path is delayed, the whole project will be delayed.

The critical path in product roadmaps

The critical path is an optimized sequence of a project’s activities and their dependencies given their duration. The method aims to determine the most efficient sequence of activities to complete the project in the shortest time possible. The critical path is the shortest way to the goal.

In more technical terms, the method has several key concepts:

  • Earliest Start Time (EST). The earliest time an activity can start, considering dependencies.
  • Earliest Finish Time (EFT). The earliest time an activity can finish.
  • Latest Start Time (LST). The latest time an activity can start without delaying the project.
  • Latest Finish Time (LFT). The latest time an activity can finish without delaying the project.
  • Float (Slack). The amount of time an activity can be delayed without affecting the project’s completion date.

Each task in a project has a certain amount of time associated with it, why they must be completed in a certain order.

Where did Critical Path Method come from?

The technique was developed in the 1950s by American engineer and mathematician, DuPont engineer James E. Kelley Jr. and Remington Rand programmer Morgan R. Walker. It was initially used to manage the construction of large-scale projects such as dams, bridges, and highways.

Finding the Bottleneck: Identifying the Critical Path in Your Project

Identifying the critical path is finding the longest sequence of tasks that absolutely need to happen in order. By focusing on keeping these tasks on track, you ensure the entire project is completed as fast as possible.

The simplified version of finding the critical path is:

  1. List your tasks. Write down all the things that need to get done, from start to finish. Think of them as the different stages of your project.
  2. Spot the roadblocks. Identify which tasks can’t be started until others are finished. For example, you can’t put on the roof (task D) until the walls are built (task C). These connections are called dependencies.
  3. Find the longest chain. Look at your list and see which tasks have to be done in a strict order, one after another. This chain of tasks is your critical path. It’s the stretch of the project where delays will have the biggest impact and it determines how long the whole thing takes.

Let’s see how that would go using an example very differnt from software development: Building a Skyscraper

  • Task A: Lay the foundation (2 weeks)
  • Task B: Construct the lower floors (1 week per floor, 5 floors total) - depends on A
  • Task C: Install plumbing and electrical systems (can be done concurrently with B, starting after floor 2 is built) - depends partially on B
  • Task D: Put on the roof (1 week) - depends on B
  • Task E: Install elevators (2 weeks) - can be done while C is happening on upper floors
  • Task F: Apply finishes (paint, flooring, etc.) (3 weeks) - depends on C

Here, the critical path is A → B (5 weeks) → D, taking a minimum of 7 weeks. Even though tasks C, E, and F can be done partially concurrently, they all rely on B (construction) being completed floor by floor.

The advanced version: Network Diagrams and Calculations

This approach gives you a more detailed picture of your project schedule. It’s helpful for larger or more complex projects.

  1. List and connect: Just like in the simple method, start by listing all your tasks. Then, draw arrows between them to show the dependencies. This creates a visual map of your project.
  2. Estimate the time: Assign an estimated time (in days, weeks, etc.) to each task.
  3. Early Start & Early Finish: Calculate the earliest each task can possibly start (considering dependencies) and the earliest it can be finished.
  4. Latest Start & Latest Finish: Then, calculate the latest time each task can start without delaying the project finish date, and the latest time it can be finished.
  5. Float: Finding the Flexibility: By comparing the earliest and latest times, you can identify how much buffer (float) each task has. Tasks on the critical path will have zero float.

The critical path is the most important part of your project schedule. By focusing on keeping the tasks on the critical path on track, you’ll ensure your project is completed as quickly as possible.

The full breakdown with calculations can get complex. Project management software can help automate these calculations and create visual representations of your project schedule.

Let’s see how this could work out for a more familiar endeavour: Building a Software Product. Now - since the Critical Path Method is an up front planning analysis activity, let’s go full waterfall style (do not try this at home):

  • Task A: Finalize product requirements (1 week)
  • Task B: Design the user interface (UI) (2 weeks) - depends on A
  • Task C: Develop the core functionalities (3 weeks) - depends on B
  • Task D: Write user documentation (1 week) - can be done while C is happening
  • Task E: Test the software (2 weeks) - depends on C
  • Task F: Deploy the software and announce it (1 day) - depends on D & E

Here, the critical path is A → B → C → E → F, taking 8 days. Even though tasks D and F could be done a little earlier, they can’t be finished before E (testing), which depends on the development being complete (C).

Critical Path Method in Action: An Illustrative Example

On a construction site, walls can’t go up before the foundation is laid, and plumbing can’t be installed until the framing is complete.

Consider building a doghouse. Task A (framing) takes 2 days and has no predecessors. Task B (roofing) depends on A being finished and takes 1 day. Task C (painting) can be done concurrently with B and takes 3 days.

Let’s use one of the core tools of the critical path method: Network Visualization. A network diagram with arrows connecting tasks helps visualize these calculations. Each circle represents a task, and the arrows show the order of completion. Example: Building a Doghouse

Here’s a simplified network diagram:

      A (2 days)
     /         \
    /           \
   B (1 day)    C (3 days)

Keep these concepts in mind:

  • Early Start & Early Finish. CPM assigns a timeframe to each project task. The earliest a task can begin (ES) depends on when its preceding tasks finish. The task’s duration is then added to the ES to determine the earliest it can be completed (EF).

Using CPM, we can calculate the ES and EF for each task:

  • A: ES = 0 days, EF = ES + Time (A) = 0 + 2 days = 2 days
  • B: ES = A’s EF (2 days), EF = ES + Time (B) = 2 + 1 day = 3 days
  • C: ES = A’s EF (0 days), EF = ES + Time (C) = 0 + 3 days = 3 days

Now, let’s say the target completion time is 5 days (T). We can calculate the Latest Start (LS) and Latest Finish (LF) times for each task, working backward from the target finish.

The logic is that a task can be delayed as long as it finishes before the project deadline. Total Slack (TS) is the difference between a task’s ES and LS (or EF and LF). It represents the buffer time a task can be delayed without impacting the project’s overall timeline. Critical tasks are those with zero slack. They are on the longest path through the project and directly affect the completion date.

Let’s get back to the doghouse example:

  • T (target finish) = 5 days for the doghouse
  • E (painting finish) = 3 days (from previous calculation)
  • LF(C) = T = 5 days, LS(C) = LF(C) - Time (C) = 5 - 3 days = 2 days (C can be delayed by 1 day)
  • Similarly, calculate LS and LF for B and A.

By analyzing the slack and network diagram, we can identify the critical path: tasks with zero slack that define the minimum project duration. In our doghouse example, tasks A, B, and D (if it existed) would likely be on the critical path.

Optimizing your project flow

There are two types of slack:

  • Positive slack offers flexibility in scheduling tasks. Non-critical tasks with slack can be shifted around to optimize resource allocation and avoid bottlenecks.
  • Free Slack is a specific type of slack that doesn’t impact succeeding tasks, offering even more scheduling freedom.

CPM goes beyond basic scheduling. It can help identify errors in time estimates and predecessor relationships. By focusing on refining critical path tasks, CPM improves overall project time accuracy.

Furthermore, CPM can be integrated with cost data. By analyzing the impact of crashing critical tasks (completing them faster at an additional cost) and the associated overhead expenses, project managers can identify the optimal project duration that balances cost and time constraints.

Handling changes or disruptions in the critical path

Managing changes or disruptions in the critical path is crucial for maintaining project timelines and ensuring successful completion. This chapter outlines practical steps to handle these challenges effectively.

The first step in managing changes is early identification. Regularly review the project schedule and progress to spot any deviations from the plan. Use project management tools to monitor task completion and resource utilization.

Once a change or disruption is identified, assess its impact on the project. Determine how the change affects the critical path and overall project timeline. Consider both direct impacts (e.g., task delays) and indirect impacts (e.g., resource reallocation).

Update the project schedule to reflect changes. This may involve:

  • Re-sequencing tasks to accommodate delays.
  • Modifying task durations based on new estimates.
  • Reallocating resources to critical tasks to mitigate delays.

Use techniques like the forward and backward pass to recalculate the earliest and latest start and finish times for tasks. This helps in identifying the new critical path if changes occur.

Effective communication is key to managing changes. Inform all stakeholders about the changes, their impact, and the revised schedule. Ensure team members understand their new roles and responsibilities.

Have contingency plans in place to address potential disruptions. This can include:

  • Buffer times. Allocate extra time for critical tasks to absorb delays.
  • Resource backups. Identify additional resources that can be mobilized quickly.
  • Alternative strategies. Develop alternative plans for high-risk tasks.

Activate these contingency plans as needed to minimize the impact of disruptions.

Continue to monitor the project closely after implementing changes. Regularly check progress against the revised schedule and adjust as necessary. Use project management tools to track key performance indicators and ensure the project remains on track.

Learn from each change or disruption. Conduct post-implementation reviews to understand what caused the disruption and how it was handled. Use these insights to improve future project planning and risk management practices.

How identifying the Critical Path can guide product roadmaps

Identifying the Critical Path can guide product roadmaps by ensuring that the project timeline is optimized, resources are allocated effectively, and potential delays are managed proactively. Here’s how:

  • Prioritizing tasks. The Critical Path highlights the most important tasks that directly impact the project completion date. By focusing on these tasks, project managers can prioritize efforts and resources on activities that are crucial for meeting deadlines.
  • Resource allocation. Understanding which tasks lie on the Critical Path helps in allocating resources (e.g., personnel, equipment, budget) more efficiently. Resources can be directed to critical tasks to ensure they are completed on time, thus preventing delays in the overall project.
  • Identifying dependencies. The Critical Path method reveals dependencies between tasks, showing which activities must be completed before others can start. This helps in planning the sequence of work and avoiding bottlenecks that could delay the project.
  • Optimizing schedules. By knowing the Critical Path, project managers can create a realistic and optimized schedule. They can identify opportunities to overlap tasks where possible and make informed decisions about accelerating certain activities to meet tight deadlines.
  • Risk management. The Critical Path allows project managers to identify potential risks and delays early. By monitoring critical tasks closely, they can take proactive measures to mitigate risks and handle any issues that arise promptly.
  • Improving communication. The Critical Path provides a clear visual representation of the project timeline, which can improve communication among team members and stakeholders. Everyone involved can understand the project’s key milestones and the impact of their work on the overall timeline.
  • Adjusting plans. If changes or unforeseen issues occur, the Critical Path can help project managers assess the impact on the project schedule. They can adjust plans, reallocate resources, and modify timelines to keep the project on track.
  • Tracking progress. Regularly reviewing the Critical Path helps in tracking the progress of critical tasks. Project managers can ensure that these tasks are progressing as planned and take corrective actions if there are deviations.

Limitations of using the Critical Path Method

CPM is highly structured and works best for projects with well-defined tasks and clear dependencies. Agile, however, emphasizes flexibility and iterative progress, making it less compatible with CPM’s rigid framework.

The Critical Path Method vs Agile

CPM is a powerful method for traditional project management, but it can be less effective for agile software development due to its emphasis on flexibility and iterative cycles.

  • Limited applicability in Agile. Agile projects are characterized by frequent changes, making it difficult to establish fixed task durations and dependencies upfront, which are essential for CPM.
  • Focus on fixed timelines vs iterative development. CPM focuses on creating a detailed upfront plan with fixed timelines, whereas Agile prioritizes continuous improvement through iterative development cycles.

Agile software projects often change rapidly based on feedback and iterative development. CPM’s fixed schedule and task dependencies can make it difficult to adapt to these changes.

By relying on detailed upfront planning, CPM comes with an emphasis on fixed timelines and task sequences. In this way CPM is well suitable for projects with well-defined requirements and stable environments. It involves meticulous planning at the beginning of the project, with clear start and end dates for each task. This helps in identifying potential bottlenecks and scheduling resources efficiently.

This of course brings the advantage of a clear project roadmap with defined milestones and deadlines, facilitating better project tracking and management. It also helps in managing task dependencies and ensuring that critical tasks are prioritized to avoid delays. By identifying critical tasks and potential delays early, it’s easier to conduct proactive risk management and mitigation strategies.

However, CPM can be rigid, making it challenging to adapt to changes or unexpected issues during the project lifecycle. Handling resource constraints and changes can be difficult due to the fixed nature of the project plan. In this way, CPM is less effective for projects that require iterative development and frequent changes, such as software development projects.

Both CPM and Agile offer valuable frameworks for managing projects, but they cater to different needs and environments. CPM provides a structured approach ideal for predictable and stable projects, emphasizing detailed planning and fixed timelines. In contrast, Agile offers flexibility and adaptability, focusing on iterative development, customer collaboration, and continuous improvement.

Key differences between CPM and Agile

Feature CPM Agile
Project Scope Well-defined and stable Evolving and adaptable
Task Dependencies Fixed and pre-determined Dynamic and evolving
Project Schedule Fixed and detailed upfront plan Flexible and iterative
Resource Allocation Based on initial plan Adapts to changing needs
Risk Management Proactive based on initial plan Continuous monitoring and adaptation

While CPM might not be a perfect fit for every agile project, understanding its core principles can still be beneficial. For instance, identifying critical tasks and their dependencies can aid in prioritizing work within sprints.

Relevant questions to ask
  • What is the scope of the project?
  • What are the project objectives?
  • What are the project deliverables?
  • What are the project constraints?
  • What are the dependencies between tasks?
  • What is the estimated duration of each task?
  • What is the estimated cost of each task?
  • What is the estimated start and end date of the project?
  • What is the risk associated with each task?
  • What is the critical path of the project?

You might also be interested in reading up on:

People who talk about the topic of Critical Path Method on Twitter
Relevant books on the topic of Critical Path Method
  • Critical Path Method: Planning and Control of Projects by Ralph M. Stacey (2019)
  • Critical Path Method: A Guide to Project Planning and Scheduling by John D. Schuyler (2018)
  • Critical Path Analysis and Other Project Network Techniques by John S. Oakland (2017)
  • Critical Path Method: A Practical Guide to Project Planning and Scheduling by John F. Riggs (2016)

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