Critical Path Analysis: Techniques for Project Scheduling

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Image of a maze depicting a projects critical path.

What's on your critical path? Important tasks? Sure. Really important tasks? Of course. But is that all there is to it?  After all, every project task is important in some sense.  Within any project, the critical path is more than just a series of important tasks - it's a means for scheduling and management that relies on connections and consequences as a basis for planning project tasks and timelines.

As a project management technique, critical path analysis provides value in four (4) key respects:

  1. The critical path analysis is a means to estimate overall project duration.
  2. The critical path analysis is a basis to create a logical sequence of project tasks.
  3. The critical path analysis is a means to track project progress and identify delays.
  4. The critical path analysis is a basis to identify potential "fast-tracking" possibilities.

Practical Steps for Critical Path Creation

Critical path tasks are not considered "critical" on the basis of value or visibility, but on the basis of dependencies, which determine the overall length of the project. Since critical path tasks are connected tasks, a delay in one, can lead to a delay in all. As such, once identified, the critical path shows you what how to get your project done on time. 

Read more:  Project Scheduling Strategies

Critical path analysis relies on a few simple assumptions, as listed below:

  • Assumption #1 - Projects are made up of tasks.
  • Assumption #2 - Tasks are combined to form a timeline.
  • Assumption #3 - Within this timeline, tasks are either concurrent (can occur simultaneously) or sequential (one task cannot begin until the predecessor [superior task] is complete).
  • Assumption #4 - Sequential, dependent tasks make up the critical path.

Finding Your "Critical Path"

Critical path analysis begins with a task list, identifying all the key tasks required to complete the project at hand. This task list can be broken down into the following seven (7) key elements:

  • Tasks: Specific work activities.
  • Predecessors: Tasks that must be completed before any subsequent, dependent task can begin.
  • Durations: Task time estimates (from start to completion).
  • Early Start Time: The earliest point in the schedule at which a task can begin.
  • Early Finish Time: The earliest point in the schedule at which a task can finish.
  • Latest Start Time: The latest point in the schedule at which a task can start without causing a delay.
  • Latest Finish Time: The latest point in the schedule at which a task can finish without causing a delay.

From a practical standpoint, critical path analysis is all about creating "breathing room", to identify the tasks that must start and end at a specific point in time, versus those tasks which offer scheduling flexibility. Any worthwhile project management software will calculate critical path for you based on the tasks, dates and dependencies entered, but the logic behind these calculations should not be a total mystery, for it is the human element that must respond to project issues and changes on a daily basis - in real time.

Read more:  Project Management Checkpoints and Fundamentals of the Work Breakdown Structure.

Critical path analysis looks for the earliest and latest points at which tasks can begin and end. The calculation of earliest start times (EST) and earliest finish time (EFT) is used to create the project schedule. The calculation of latest start times (LST) and latest finish times (LFT) is used for schedule management, delay resolution, and fast-track planning.


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Calculations: Early Starts and Finishes

EST of tasks with no predecessors = First logical starting point.

EST of tasks with predecessors = Predecessor EFT (Earliest Finish Time).

EFT of tasks with no predecessors = Estimated task duration.

EFT of tasks with predecessors = (Task EST + Estimated task duration).

On the other side of the coin, latest start (LST) and latest finished times (LFT) are backwards calculations, considering the earliest starting point of the first subsequent task, minus the expected duration of the task under calculation. To calculate LST and LFT, you will start with the latest finish time and work backwards to calculate the latest possible start time.

Calculations: Late Starts and Finishes

Step 1 - Finding the LFT (latest finish time):

Considering the estimated "earliest start time" of any subsequent dependencies, what is the latest finish time for this task?

Task LFT = EST of the first dependent task.(Example: LFT of Task 1 = EST of Task 3)

Step 2 - Finding the LST (latest start time):

Considering the identified "latest finish time", what is the latest starting time for this task?

Task LST = (LFT – Task duration).

Example: LST of Task 1 = (Task 1 LFT - Task 1 Duration).


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