Module 2: Critical Path Explained
This module details the Critical Path Method — calculating the critical path and how it can be used to assess and analyse project schedules. Before we work through the calculations, some common technical terms need to be defined.
Key Technical Terms
Activity
A distinct item of work, completion of which is necessary to finish the project. Also called a task.
Duration
A realistic, best estimate of the time necessary to complete an activity. Measured in time units (e.g. days).
Early Start (ES)
The earliest point in time when an activity can start, assuming all prerequisite activities take place as scheduled.
Early Finish (EF)
The earliest date that an activity can finish, assuming it began on its early start date.
Late Start (LS)
The latest point in time an activity must start if there is to be no delay in subsequent activities.
Late Finish (LF)
The date by which all work in an activity must be completed in order to avoid delaying subsequent activities.
Critical Path
The sequence of activities from project start to finish that has the least amount of float — requiring the longest total time to complete. Any delay to a critical path activity delays the project end date.
Longest Path
The sequence of activities with the highest total duration. It may or may not be the critical path.
Total Float
The amount of extra time available to an activity (or activities on a path) when all activities start as early as possible. Also called slack.
Free Float
Float uniquely available to an activity. It indicates the duration an activity can be delayed without delaying its successor tasks.
Slack
Another term used for Float.
Task
Another term for an activity.
Zero Float
Denotes activities that have no flexibility — they must start and finish on or before their scheduled dates for the project to be completed on time.
Negative Float
Denotes activities scheduled to occur after their late dates. The project is delayed. Negative float is a warning — the schedule must be reviewed and corrective action taken.
In the next lesson we look at Lesson 2: Scheduling Methods — Bar Chart and CPM scheduling, how the critical path is calculated manually using forward and backward pass, and a comparison of the three major CPM software packages.