A network diagram is the diagram used in Critical Path Analysis (CPA).
Critical Path Analysis (CPA) relies on network diagrams to map out the logical sequence of activities and their logical dependencies between.
These diagrams provide a visual representation of the project’s overall flow, making it easier to identify the critical path – the sequence of tasks that determines the minimum project completion time.
Elements of a network diagram
A network diagram can help with monitoring and controlling the execution of the project, breaks down a complex project into activities, identifies activities that must be completed on time to ensure the project is not delayed.
A network diagram uses the following notation:
- ARROWS. An arrow indicates each activity. It denotes an activity which has a duration. An activity takes up time and resources. Each activity appears only once, with arrows depicting the flow of activities and their durations. No activity starts and ends at the same node.
- NODES. A node (circle) indicates the end of each activity. It denotes an event, the start and finish of each activity. Activities with no preceding tasks begin at a common node (project start). Conversely, all activities with no successors converge at another node (project end).
Network diagrams aid in tracking project execution and identifying potential roadblocks.
Understanding Earliest Start Time (EST) and Latest Finish Time (LFT)
By understanding network diagrams and their elements like Earliest Start Time (EST) and Latest Finish Time (LFT), project managers can leverage Critical Path Analysis (CPA) to effectively plan, monitor, and control project execution.
1. What is the Earliest Start Time (EST) and how is it calculated?
Earliest Start Time (EST) of each activity is the earliest time each activity can begin, taking into account all of the preceding activities. If there is more than one activity starting at the node, select the larger number. It is the earliest time an activity can start.
For example, Activity B cannot start before Day 10 because the first Activity A will not be finished before then. And, Activity C cannot start before Day 15 because both Activity A and Activity C have to be completed first.
The easiest way to calculate the EFTs is to work from left to right. Where there is a choice of routes forward to a node, the aim is to achieve the highest number for EST. Remember, the highest number at each node is what is required for the EST.
2. What is the Latest Finish Time (LFT) and how is it calculated?
Latest Finish Time (LFT) of each activity is the latest time an activity must finish so that the entire project can finish within minimum duration time. If there is more than one activity starting at the node, select the smaller number. It is the latest time an activity can finish without delaying the whole project.
For example, Activity Y (and all preceding activities) must be finished by day 50 or the entire project will take longer than 60 days as 10 days must be allowed to finish the last Activity Z itself.
The easiest way to calculate the LFTs is to work from right to left. Where there is a choice of routes back to a node, the aim is to achieve the lowest number for LFT. Remember, the lowest number at each node is what is required for the LFT.
3. What is the critical path for this project?
The critical path is the longest path through the network indicating the shortest time within the project can be finished.
All activities on the critical path, or critical activities, have no spare time – no delays possible. They must start and finish exactly on schedule. It is because if a critical task is delayed, the entire project will be delayed. The critical activities are usually marked with a different color or a double slash on the chart.
The project duration is the sum of durations of all critical activities.
4. How to calculate float times for non-critical activities?
Non-critical activities, or activities which are not on the critical path have float time. Float times have very significant applications in managing project resources. It means that they will have a certain amount of spare time (float). There are two types of float:
a. Free float. The amount of time an activity can be delayed without delaying the start of the following activities. Or, not delaying the EST of the next activity. Free float tends to be more appropriate when the delivery of materials must be on time or when labor is involved in other activities and cannot be moved on to another job. This is calculated by the formula (in days):
Free float = EST (next activity) – Duration of current activity – EST (current activity)
Let’s say that the EST of Activity A is Day 0 and its duration is 6 days. The EST of Activity B, which is the next activity after Activity A, is Day 20. Then, free float for Activity A equals to Day 20 – 6 days – Day 0 = 14 days. This means that Activity A could be delayed by 14 days without delaying the start of the next Activity B.
b. Total float. The amount of time an activity can be delayed without delaying the whole project. Or, how long an activity can be delayed, so that the project is still completed within minimum duration time. This is calculated by the formula in days:
Total float = LFT (current activity) – Duration (current activity) – EST (current activity)
Let’s say that the EST of Activity B is Day 20, its duration is 8 days and the LFT is Day 32. Then, total float for Activity B equals to Day 32 – 8 days – Day 20 = 4 days. This means that Activity B could be delayed by up to four days without delaying the project duration, extending the total project duration or changing the critical path.
5. What to do with dummy activities?
Showed as a dotted line, a dummy activity is not an activity at all (as indicated by nodes), but shows a logical dependency between other activities included in certain networks. It prevents an illogical path from being created. Dummy activities do not consume either time or resources.
For example, Activity A and Activity B are at the start of the project. They have no preceding activities. Activity C follows Activity A, and Activity D follows both Activity A and Activity B. It would be wrong to draw the network diagram in the following way:
It is because the network above shows that both Activity C and Activity D require Activity A and Activity B to be finished, whereas only Activity C requires Activity A to finish before it can start. The correct network is shown below:
It is because it shows the correct logical dependencies meaning that Activity C starts when Activity A is finished, but Activity D has to wait until both Activity A and Activity B are finished. The dummy activity shows the logical relationship between Activity B and Activity D with the arrow indicating the direction of the dependency.
In summary, a network diagram is a fundamental tool in Critical Path Analysis (CPA). It visually represents the project activities and their dependencies, forming the foundation for scheduling and identifying the critical path.
