While most project professionals are aware of time scheduling concepts, the finer details may seem a little confusing to those who have not completed specialist planning and scheduling training.
In this short article aimed at Project Managers, Project Engineers, Quantity Surveyors and other project professionals, I will explore the key elements of Time Analysis and more specifically, the Critical Path Method.
The aim of Time Analysis is to calculate the boundaries in which each activity in the network can take place. This is done through the assessment of activity parameters such as estimated durations, interdependencies or resources.
There are two types of scheduling: Critical Path and Critical Chain
- The Critical Path Method (CPM) focuses on the analysis of activities in order to calculate two sets of dates (Early Finish and Late Finish for each activity). Those calculations determine time boundaries in which the works can be completed without delaying the project completion date.
- The Critical Chain Method focuses on the analysis of resources, aiming to keep resources at constant utilisation. It is also sometimes called Resource Critical Path.
The Critical Path Method
The CPM is defined in the APM Project Management Body of Knowledge (PMBOK) as follows:
“The Critical Path Method is an activity based scheduling technique that determines the overall duration of the identified work based on estimates and logical dependencies. The method of determining the Critical Path.”
Critical Path Method has embedded itself firmly in the theory and practice of project management. It is widely adopted at all stages of project lifecycle. You will see it used to support tendering efforts, on site to manage the works, in commercial discussions surrounding obligations and entitlement.
Critical Path Analysis is almost always performed using specialist scheduling software, such as Microsoft Project, Asta Powerproject, Primavera P6 or other less popular such as Spider Project or Safran (the above 2D tools are increasingly more often combined with or replaced by 4D modelling tools but for now, let’s focus on 2D modelling).
The schedules produced using the tools available on the market will be presented as Gantt Charts (that we are all familiar with), Time-Location/Time Chainage Charts, Line of Balance Graphs (those may not be as popular as Gantt Charts but certainly have their use in various industries), etc.
In order for any analysis to take place, we must first establish the Critical Path. Generally, it happens in 5 steps listed and further described below.
- Step 1 – Create a logical network
- Step 2 – Forward Pass
- Step 3 – Backward Pass
- Step 4 – Calculate Total Float
- Step 5 – Identify Critical Path
Step 1 – Create a logical network
The process starts with spinning the web of activities. At this step activities required to complete the project are identified along with the dependencies between them. Each activity is assigned a time estimate based on the effort required to complete the task.
Step 2 – Forward Pass
The Forward Pass calculates the EARLIEST Start and Finish Dates for each activity based on the information established in Step 1. The calculations start on Day 1 assigned to the first activity in the chain and progress through the network aiming to establish the earliest possible Finish Date for the whole network.
Early dates can be used to create the Early Curve, or in other words identify the P0* plan. P0 indicates that the probability of achievement of such plan is zero percent. Such plan represents perfect delivery, with absence of risk and with the availability of unlimited resources.
Step 3 – Backward Pass
The Backward Pass calculates the LATEST Start and Finish dates for each activity. The calculations start with the last activity in the network when its Latest Finish is set to the same value as the Early Finish established via the Forward Pass. Then, in a nutshell, this process is the reverse of the Forward Pass, working backwards through the network.
Late dates can be used to create the Late Curve. Working to the late curve is theoretically possible but means that absolutely no deviation from the plan is allowed as any deviation will cause delays and overspend based on the existing assumptions.
Step 4 – Calculate Total Float
Once the Forward Pass and the Backward Pass are complete, Total Float can be calculated. Latest Finish less the Early Finish equals Total Float for the activity and is expressed in the relevant time unit (most often hours or days).
For more information about types of float and float ownership see Float and Float Ownership.
Step 5 – Identify Critical Path
The chain of activities with zero float becomes the Critical Path. Very often in scheduling software at this point critical activity bars and the logic links that connect them change their colour to red.
Please note that as an exception to the rule, in some cases the Critical Path may not be defined by the scheduling software as the chain of activities with zero days of Total Float. One such example is when different calendars are used throughout the network.
One would assume that the steps above will result in a workable plan
At this stage, although the schedule is theoretically possible to work to, in real life terms it is unachievable as it deals with extreme values. CPM is usually used in conjunction with other techniques and tools.
This brings us into the realms of probabilistic planning, business strategies, risk management, and others, that we will explore in future parts of this guide.
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APM’s Planning, Scheduling, Monitoring and Control (2015)
APM Body of Knowledge 7th Edition (2019)