It’s just a little over 40 years ago that I had to learn the basics of CPM the hard way. CPM – Critical Path Method, is a powerful but basically simple technique for analyzing and scheduling projects. Fresh in the planning department I had to teach myself two new skills; how to use the planning system and the basics of the CPM algorithm. We were about to deliver the baseline of a new engineering project and had only a few weeks until deadline.
Over the years several books and articles have been written about the critical path method, its basics, use and benefits. To understand schedules, project managers and schedulers need to understand the basics of scheduling terms, and to fully understand these terms they need to understand how their values are calculated. The importance of scheduling can only be understood if you become knowledgeable in the basics of CPM and the schedule calculations.
Planning and Scheduling
Planning and scheduling are terms that go hand in hand in the project management literature, and they are often used interchangeably so that planning and scheduling become the same. We talk about the planning department and the plan. The planning department is busy building, updating and maintaining the schedule and present the schedule as “The Plan.” However, we should note that the developers of the critical path method separated the two tasks.
“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).
CPM is dealing with the scheduling issue. Your plan or strategy to reach the goal is a prerequisite to the creation of the (time) schedule.
The Essentials of CPM Scheduling
The critical path method consists of a series of calculations to determine the start and finish dates of activities, the end date of the project, and the critical path of your schedule if no delays occur. The “if no delays occur” is essential to understanding the scope of the scheduling. “If no delays occur” can be translated into ”if everything goes according to plan”.
The CPM is built around activities, durations, and the logic relationships between the activities. What is known as a network, or network of activities. When you build the network, you must decide which activities must precede, which must follow, and which activities can go in parallel.
Figure 1: The activities and their relationships form a logical network.
The CPM calculations can be broken down into three steps:
The forward pass: Starting with the first activity and working its way through the network logic to calculate the earliest starting dates and earliest finish dates for each activity.
The backward pass: Starting at the end date of the last activity in your network of activities and working its way backwards to calculate the latest finish date and latest start date for each activity.
Float calculations: The last part of the CPM analysis is to calculate the float or slack for each activity in the network. Float is used to find the critical path, where no delays can occur without affecting the project end date, and how much other activities can be delayed without affecting the end date or the start of the succeeding activities.
Float is an important asset to the schedule and the project as it gives you some wiggle room and allows you to move some of the activities (for example to accommodate resource restrictions) without affecting the end date. It can also serve as buffers if delays occur.
Critical vs Non-Critical
You may hear that critical activities are those that you identify as important and include in the critical path. No! Important does not equal critical. It is a misunderstanding of the concept of CPM and the term critical. Many activities that are identified as important will not be on the critical path. To complete the project successfully, you could argue that all activities are equally important.
Multiple Critical Paths
In complex projects, you can have multiple critical paths between the project start and project finish milestones. It is possible to have multiple parallel and equally long paths through your network. Multiple critical paths can also be the result of dangling activities or open ends, that is activities with no successor.
The critical path to a project is not static. As estimates for activity duration change or as work takes longer or less time than planned, the critical path may shift either in time or to an new sequence of activities. During execution and when updates is made to your project the critical path may switch to another path of activities.
Activity Dependencies
The simplest, the most used and probably the type of logic between two activities that is easiest to understand is the simple Finish-start type. Basically, this states that when I’m finished with activity A I can start with activity B. Or I cannot start activity B before Activity A is finished.
In the project management literature, you may find dependencies, constraints, links, or logic used as terms to describe the relationship between activities.
There are 4 different types of logic that can be used.
Finish – Start (FS): Activity A must be completed before you can start activity B.
Start – Start (SS) When activity A has started, activity B can also start in parallel.
Finish – Finish (FF) Activity A must finish before activity B can finish.
Start – Finish. When activity A starts, Activity B can finish.
A start-to-start relationship allows activities to run in parallel, as do the Finish-to-finish relationship. The figure above show the difference between the 4 types of relationship and their effect on the succeeding activities.
Recommended practices on scheduling are that all activities should have at least one predecessor (except for the start activity) and at least one successor (except for the finish activity). The use of multiple types of logic adds more complexity to the calculations The majority of relationships within a detailed schedule should be finish-start.
Non-Working Periods
In most countries the normal work week is 5 days excluding week ends. Holidays and vacation will also determine when work effort can and cannot be scheduled. Work may take place at different locations, different regions, and different countries, meaning that you must take different calendars and non-working periods into account when calculating the start and finish date of each activity and the critical path of your project.
The above Barchart shows the effect of working and non-working periods and holidays. The first example shows a continuous work pattern with no non-working periods. In the second example there are non-working periods during weekends (day 6&7 and day 13&14). In addition, a mid-week holiday is marked as non-working. The effect is that the finish date is extended from the end of the 10th working day until end of day 15.
Terms
The following technical terms need to be understood in relations to the Critical Path Method.
| Activity | Sometimes referred to as Task. An activity is an element of work that must be performed in order to complete a project. An activity consumes time and must have resources associated with it. |
| Backward pass | CPM network calculation that determines the latest each activity can start and finish and still maintain the duration of the project as calculated by the forward pass. It calculates backwards starting from the early finish of the latest activity to determine the latest possible start and finish date or time that will not delay the project. |
| Duration | Best estimate of the length (continuous time) needed to complete the activity. |
| Calendar | Defined work periods, rest periods and holidays that determines when project activities can be scheduled. Multiple calendars can be used for different activities. This allows for more accurate modelling of the schedule. |
| Critical activity | An activity on the critical path. Activities with no float or negative float. |
| Critical path | The path of critical activities from start until finish of your project, or through your schedule to a schedule milestone such as interim milestone, substantial completion or the final completion. |
| Early start | The earliest possible start date or start time of an activity. Calculated by the schedule during the forward pass. |
| Early finish | The earliest possible finish date or time and activity may finish as calculated by the forward pass. Equals to the Early start of the activity plus the (remaining) duration. |
| Forward pass | Determines the earliest possible start and finish date or time for each activity based on logical constraint sand durations. |
| Late start | The latest date or time an activity can start so the project can finish on time as calculated by the backward pass |
| Late finish | The latest date or time an activity can finish as calculated by the backward pass. |
| Logic | Also referred to as sequencing, linking, dependency, and relationship. The relationship defines how activities relate to one another and establishes the scheduling logic. Every activity (except for the start activity) should have a predecessor, and every activity (except for the finish activity) should have a successor. |
| Network | The series of activities required to complete a project or a part of the project. A network typically consists of the activities and events that must be accomplished to reach the objectives, showing their required sequence of accomplishments and interdependencies. |
| Network Analysis | Process of calculating the start and finish dates of each activity by the use of a backward pass and a forward pass through the CPM model. |
| Free Float | The maximum amount of time an activity can be delayed without delaying the succeeding activity. |
| Total Float | The maximum amount of time an activity can be delayed without delaying the finish date of the project. |
| Path | A continuous chain of activities within a network |
| Predecessor | An activity that precedes the current activity |
| Preceding activity | An activity that immediate precedes the current activity |
| Preceding path | The common path(s) or chain(s) of activities preceding the current activity. |
| Successor | An activity that succeeds the current activity |
| Succeeding activity | An activity that immediate succeeds the current activity |
| Succeeding path | The common path(s) or chain(s) of activities succeeding the current activity |
The above list of terms is just an extract from the terminology used in the project management discipline. If you want to study more comprehensive lists of glossary and terms used, you can look up these resources:
- AACEi website 10S-90: Cost Engineering Terminology (aacei.org)
- APM website Project management Glossary | Terms used by project professional (apm.org.uk)
- Vocabulary ISO/TR 21506 published - IPMA International Project Management Association
You may also want to look up the ISO/TR21506 Technical Report. A good starting point can be found at the IPMA website.
Recommended next reads:
Why Is your Critical Path So Critical?
12 Best Practices for Preventing Project Schedule Overruns
