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Your schedule tells you when your project will finish if there are no delays. It’s the critical path that determines the length of your project. Project schedulers and project managers should know and understand which activities make up the critical path as it is key to understanding how delays along this path will delay the project. In project management terms the critical path is often defined as “the sequence of activities that determines the duration of the project.”
Simply explained, the critical path is all the activities that determine the end date of your project schedule. If one of those activities is delayed or late by one day, then your project end date will be extended by one day. Critical activities do not have any spare time available.
Not all tasks are equally critical
Several project activities will not be on the critical path. This is due to slack or float in the project schedule. Slack or float is the amount of time an activity can slip or be delayed without delaying the end date of the project. Any delay along the critical path will make the project longer. Delays that are not on the longest path will not delay the project unless the delays are so great that it makes that path critical. During project execution, delays are likely to occur, and the critical path may change. Any delays and deviations should be watched closely.
If you want to shorten the project duration, you can only do so by shortening activities that are on the critical path. If you shorten activities that are not on the critical path, the project duration will remain unchanged. If you shorten the critical path a new path may become the new critical path determining the length of the project.
If the critical path runs through your admin and management activities, you should take a closer look at your scheduling.
The critical path method, in addition to calculating the start and finish dates of each activity and the entire project, also calculates the spare time for each activity. In scheduling terms this is referred to as float.
Float
Float or slack is the amount of time an activity can be delayed without delaying the entire project or the succeeding activity. Float is an important asset to any project as it provides flexibility in the schedule. Float identifies non-critical activities that can be delayed.
Total Float: the amount of spare time available and the delay that can occur without affecting the project completion data.
Free Float: it measures the time an activity can be delayed without upsetting future early times.
You can read more about float in this blog from 2020: What is Project Float? Distributing Slack with Safran Project.
The Origins of CPM – “see what you can do about scheduling”
Most project management literature points to the birth of modern project management having taken place in 1957, when James Kelly Jr. and Morgan Walker worked out the basic techniques and algorithms of CPM. The project management discipline is closely linked to the importance of critical path analysis. The Critical Path Method (CPM) was developed over a period of 27 months and is one of the most powerful and important concepts in project management. In his book, “Managing project as investments,” Steven A. Devaux argues “that no single technique is more important within the project management discipline.” (Steven A. Devaux (2015), Managing projects as investments p. 60).
The critical path method was developed by the Integrated Engineering Control Group (IEC) of EI duPont. A “think-tank” group and an environment for new ideas at duPont at that time.
IEC issued a survey late 1956 of the prospects to use computers as an aid to cope with the complexities of managing engineering projects. The 3 questions of most interest where: To what extent can a computer-oriented system be used:
- to prepare a master schedule for a project?
- to revise schedules to meet changing conditions in the “most” economical way?
- to keep management and the operating departments advised of project progress and changes?
Just before Christmas in 1956 the team was given the task “se what you can do about scheduling”. (Kelly, J.E., Walker, M.R., & Sayer, J.S. (1989) The origins of CPM: a personal history, PM Network 3(2), 7-22)
The model or technique that the duPont team came up with for the planning and scheduling was the Critical Path Method. “This name was selected because of the central position that critical activities play in the method.” (James E. Kelly and Morgan R. Walker (1959), Critical-Path Planning and scheduling.)
When developing the CPM algorithm, the planning task was separated from the scheduling task. “The first step in building a model of a project planning and scheduling system was to separate the functions of planning from scheduling. We defined planning as the act of stating what activities must occur in a project and in what order these activities must take place. Only technology and sequence were considered. Scheduling followed planning and is defined as the act producing project timetables in the consideration of time and cost”. (James E. Kelly and Morgan R. Walker (1959), Critical-Path Planning and scheduling).
This distinction is still valid today: “The Contract Detail Schedule activities shall reflect the intended execution strategy and Contractor’s method of performing the work.” , “The contract schedule shall demonstrate the logical sequence in which the contractor intend to carry out the work.”
"Do you have a plan or strategy to reach done?" Is one of the questions asked in “The five immutable principles of project success” by Glenn B. Allemann. You can download the guide here. We also have a webinar recording on the topic here.
Proven track record
CPM Scheduling is a powerful and widely used technique for project scheduling. CPM and the use of software supporting and built around the CPM algorithm has been successfully used on complex projects for nearly 70 years since its birth.
Focusing on the critical path help planners, schedulers and project managers manage project time. Unfortunately, in many businesses the critical path analysis is undervalued, underutilized, and poorly understood. “Perhaps what causes some to ignore critical path planning is the fact that the critical path schedule will almost never be achieved in actuality. Technical problems will arise, expected resources will be absent, there will be a gap in “work hands-off”, and the critical path itself will quite likely migrate to a completely different set of activities. But none of this lessens the benefit of critical path analysis; indeed, the more variances from plan that there are, the more valuable the critical path analysis is for measuring impacts and making decisions.” (Steven A. Devaux (2015), Managing projects as investments p. 60-61.)
Modern tools and modern planning and scheduling applications has made the scheduling part of planning and scheduling quicker and easier by providing interactive Gantt charts, real time CPM calculations and more. Unfortunately, at the same time the knowledge and understanding of the CPM fundamentals have diluted.
The information from the CPM analysis helps in addressing:
- The total time required to complete the project if no delays occur.
- The times the individual activities need to start and finish (at the earliest and at the latest) if no delays occur.
- The critical bottleneck activities where any delays must be avoided to prevent delaying the project completion time.
- The amount of delays that can be tolerated for a particular activity without delaying the project completion.
