The Guide to Critical Path Management

Everyone can benefit from learning about critical path management and how it could improve their business, regardless of the industry or size of your company.

Critical Path Management

Critical path management is one of the key principles for project management, originating in the early 1940s. This approach helps you to define the main tasks in a project that affect successful delivery. All critical activities, therefore, constitute the critical path.

“Recently added to the growing assortment of quantitative tools for business decision making is the Critical Path Method—a powerful but basically simple technique for analyzing, planning, and scheduling large, complex projects. In essence, the tool provides a means of determining which jobs or activities, of the many that comprise a project, are “critical” in their effect on total project time, and how best to schedule all jobs in the project in order to meet a target date at minimum cost.” Harvard Business Review, 1963

The critical path is the sequence of activities that represents the longest path through a project, which determines the shortest possible project duration.

When you know which tasks are important to the project and how to achieve them quickly, you took the first step in the successful delivery of a project.

You need to also be familiar with the “float” term and know why it’s the key of the  Critical Path Method.

Float Definition

“The slack or float is an amount of time an activity can be delayed by causing no delays in other dependent tasks and the project. To deliver successfully the project, critical activities should have zero floats”

Total Float:  is the amount of time that an activity can be delayed from its early start date without delaying the project finish date.

Free Float: is the amount of time that an activity can be delayed without delaying the early start date of any successor activity.

An activity with zero Total float is considered a critical activity. If the duration of any critical activity is increased (the activity delayed), the project finish date will delay.

Project managers should define what activities are completely non-flexible when preparing. Therefore, the critical path method works so well in industries where there are perfect project plans with deadlines that have precise dates and can not be delayed.

Activity-On-Node (AON)

Activity-on-node is a concept in project management that refers to a form in precedence diagramming that uses boxes to indicate schedule activities. These different boxes or “nodes” are connected with arrows from start to finish to depict a logical progression of the dependencies between scheduling activities. Each node is coded with a letter or number that corresponds to an activity scheduled for the project.

The activity-on-node diagram would usually be built to demonstrate which activities have to be performed in order for other activities to begin. It is referred to as the precedence of “finish-to-start” – meaning one task must be completed before the next one can start. Activities A and D must be done in the diagram below so that activity E can start. Many variants of that type of diagram can also be made. For example, a “start-to-start” diagram is one in which a predecessor activity simply needs to start rather than complete to begin the successor activity.


An activity-on-node diagram can be used to provide a visual representation of a whole schedule of a project’s network logic. Or, it can be used for any smaller section of the schedule that presents itself as having a defined start and end. To keep the logic in the diagram simple, the inclusion of only critical path scheduling activities may be most effective. Pursuant to the project management timeline, the expected start date of each node can also be mentioned in the diagram legend.

Relationships between Activities

Relationships are key elements in critical path project management. To build the correct order of your activities, you should designate all possible predecessors. That means you need to link every dependent activity to the activities coming before it that need to be finished before the next stages can start.

Types of Relationships:

  • Finish to Start (FS)
  • Finish to Finish (FF)
  • Start to Start (SS)
  • Start to Finish (SF)

Activity-On-Arrow (AOA)

It is a network diagramming technique in which activities are represented by arrows. The start and end of each node or event is connected to an arrow. Between the two nodes lie an arrow that represents the activity.


Early Start (ES): It is the earliest time that an activity can start with

Early Finish (EF): It is the earliest time that an activity can finish with. (E.F = E.S + Duration (D))

Late Finish (LF): It is the latest time that an activity can finish with.

Late Start (LS): It is the latest time that an activity can start with. (L.S = L.F – Duration (D))

Total Float: The total float for an activity is the amount that its duration can slip without causing the project to be delayed. Any activity with a zero float is on the critical path.

TF= LF – EF or LS – ES

Free Float: The free float is how long an activity can be delayed, without delaying the Early Start of its successor activity.

Learn more about: Free Float in Project Management

Identify the Overall Critical Path

An easy way to create and calculate a critical path is to use a software (Primavera P6, Ms. Project,etc.) that does all the work for you.

  • Add the activities
  • Add the Duration for each activity
  • Add relationships between all the activities
  • Schedule(Run) the Programme

See Also
Primavera P6 Shortcuts
Estimating the Activity Duration in Project Management