The critical path method is the most used project management technique. Learn the CPM with 7 FAQs ranging from definition to method, process, tests. The attached excel sheet (hyperlinked in the article) explains the implementation of CPM in a project schedule.

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Civilverse Admin 4 September 2021
Critical Path Explained with 7 Q&As & a Free Excel
Template
civilverse.org/critical-path
Introduction to Critical Path Method-
1. Q1: What is a Critical Path Method?
2. Q2: What is a critical path & how does it affect the project completion?
3. Q3: What terms should I know to understand the critical path method?
4. Q4: How to identify the Critical Path & calculate the project duration?
5. Q5: How to check critical path?
6. Q6: Does critical path change?
7. Q7: What affects the critical path?
8. Free Excel Template to Calculate the Critical Path for Your Project
9. Final Words

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Q1: What is a Critical Path Method?
Critical Path Method (CPM) is a project management technique used for scheduling. It
was developed in the late 1950s by Morgan Walker & E. Kelley Jr. in the USA. It is an
effective technique to identify the activities that directly affect the project duration.
A network diagram is the backbone of CPM. It is made up of activities logically &
sequentially linked to each other. Any two activities are linked by one of the below-given
relationships in a network diagram, therefore it is important to understand these links.
Finish to Start (FS): Anactivitystarts after the preceding activity ends.
Start to Start (SS): Two activities start together.
Finish to Finish (FF): Anactivity ends with another activity at the same time.
Start to Finish (SF): Successor activity ends before the start of preceding activity.
The above-mentioned links rule the project duration calculation and hence are of utmost
importance. A network diagram helps in the easy identification of activities that affect the
project completion most. What are these activities called? Read the next Q&A.
Q2: What is a critical path & how does it affect the project
completion?
A Critical path is the longest chain of activities in a network diagram that is to be done to
complete the project. The activities constituting the critical path are called critical
activities. The duration of critical activities governs the total duration of the project.
The critical path duration is the minimum time required to complete the project. Hence,
the delay in the execution of a critical activity leads to a delay in project completion. Also,
the float or slack for a critical activity is zero.

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Confused about the float? what does that mean? The next section answers your query with
all the related CPM terminology.
Q3: What terms should I know to understand the critical path
method?
Earliest Start Time (EST): The time by which an activity can commence without
affecting its predecessor. It is equal to the earliest finish of predecessor activity.
Earliest Finish Time (EFT): Thesum total of activity duration and earliest finish
time of an activity is the earliest finish time for an activity. EFT represents the time
by which an activity can end at earliest.
Latest Start Time (LST): The LST is the time by which the start of an activity can
be delayed without affecting the succeeding activity.
Latest Finish Time (LFT): It is the most delayed time by which an activity can
end without affecting its successor.
Milestone: It is a task with zero duration and marks the start or end of the project
or an important task.
Dummy Activity: Dummy activity also has zero duration but it is different from
milestone. It is added in a network to show the relationship between activities when
the relationship cannot be shown by arrows. For example, in below network
diagram, a dummy activity is added to show the dependence of activity E on activity
C.
Forward Pass: It is a process of calculating the project completion by adding the
activity durations starting from the first activity to the last in a network diagram.
The ESTs and EFTs for each activity are calculated. The longest path (critical path)
in the network diagram has the shortest duration to complete the project.
Backward Pass: It is a process of back calculating the duration of a project
starting from its end date. The LSTs & LFTs are calculated.
Float: Itis a duration by which an activity can be delayed without affecting the
project completion. The activity float is the difference between EST & LST or EFT &
LFT of an activity. As said earlier, the critical activities have zero float and hence the
EST=LST & EFT=LFT.
Activity Crash: The process ofreducing the activity duration by assigning
additional resources is called activity crash. The cost of crashing an activity
generally higher than the normal execution of an activity. It is generally used to
cover up the delay on critical path.
Q4: How to identify the Critical Path & calculate the project
duration?
We will understand the process with the help of an example. The below table contains a
list of activities to complete the project.

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Nomenclature for Network Diagram
We will follow the below-given nomenclature and format for the network diagram.
Let us create a network diagram and calculate the
EST, EFT, LST & LFT for each activity.
Network Diagram with Activity Duration
After the network diagram, we will do a forward pass with the following steps.
Forward Pass
The starting activity is assumed to start on zero date and it acts as EST for activity A.
The EFT of activity A is calculated by (EST + duration of A).
The EFT of activity A acts as EST for its’ predecessor activity (i.e., activity B).
Process is repeated for each activity.
For activity H, the EST comes from two different activities i.e., E &G. The higher
value of the two is considered.
The EFT of end activity (i.e., activity H) is the project duration (i.e., 19 days for the
given example).

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Forward Pass for Calculation of EST & EFT
Now, we will perform a backward pass.
Backward Pass
The EFT of end activity H is its’ LFT.
The LST of activity H is calculated by deducting the duration from LFT.
The LST of each activity acts as the LFT for its’ predecessor.
For activities having two successors, the LFTs are two. Take the smaller value as the
LFT of that activity.
Backward Pass for Calculation of LST & LFT
Float Calculation
The difference between early and late time is float for that activity.
Float Calculation
Critical Path Determination

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The chain of activities having zero float is the critical path for the project. In the
given example, the critical path is A-B-C-F-G-H.
Q5: How to check critical path?
Now, we have determined the critical path it is important to check if it is the correct one
before reporting it to stakeholders. For checking the authenticity of critical there are two
methods as mentioned in the DCMA 14-point schedule quality metrics. Let us discuss
them.
Critical Path Test
It is a simple test involving a critical activity. Perform this as below mentioned steps.
Choose any one of the critical activities.
Increase the duration of this activity by x number of days.
Recalculate the project duration.
Now, the project duration has also increased by x days. If so, the network diagram is
said to have passed the critical path test.
A failed critical path test indicates the missing relationship between the activities.
Critical Path Length Index
This test checks the likelihood of achieving the targeted completion date. Follow the
below-given steps to check the CPLI.
Calculate the remaining working days on critical path.
Calculate the total float.
Calculate the total remaining days.
Now, add the remaining working days and total float. Divide the obtained sum by
the remaining days. This gives the CPLI.
A CPLI of 1 indicates the project will complete on time. Value lesser than 1 indicates the
delay and the CPLI of greater than 1 show the early completion of the project than the
planned date.
Q6: Does critical path change?
Yes, it does. During the project execution, some tasks may get delayed, expedited or
experience their duration change and hence, their ETs & LTs change. The change in ETs
and LTs result in a change in float and hence the critical path.
Let us understand with the same example as explained earlier. During the 9 day of
project execution (after completion of activity B) following points were observed-
The activity B took 6 days to complete against the original duration of 5 days.
th

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The activity D will take 6 days to complete against the original planned duration of 1
day due to various reasons.
Due to the above changes, the forward and backward passes are to be re-done. Now, ETs
and LTs of the upcoming activities have changed so is their float. Following are the
changes-
Change in Critical Path
The float of activities C, F & G have increase from 0 to 2 thus, are not on critical path
anymore.
The activities D & E have become critical activities as their float of 1 day has reduced
to zero.
Q7: What affects the critical path?
If you have gone through the entire post, you must have figured out some of the factors
affecting the critical path. Here, I have summarised them.
Inter-dependency between the activities.
Duration of an activity. The uncertainty of activity duration leads to the risk of
critical path change. Also, the incorrect estimation of activity duration may generate
misguided metrics. Read these 6 powerful methods to correctly estimate the
activity durations.
Work breakdown structure. The incorrect capturing of the project scope or the
level of decomposition of scope may affect the sequencing and relationship between
the activities thus, affecting the critical path.
Resource assignment to the activity. If the assigned resource is changed or
increased/decreased or there is difference in planned and actual productivity, the
duration of activity changes.
Updating of the project. Inaccurate measurement of works or tasks missed
during the update may lead to change in the critical path.
Free Schedule Excel Template to Calculate the Critical Path for
Your Project

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Download the free excel schedule template for a residential building project. The template
calculates the ETs, LTs and the float for each activity. The critical activities are highlighted
in yellow and the separate column also tells the critical activity status. The expected
completion dates for upcoming and in-progress summary tasks are projected in the actual
finish column.
Excel Template Snapshot
In the in-built Gantt chart, the critical path is highlighted with red bars.
Download the Excel Template for Critical Path Determination
The purpose of this excel is to reinforce the concepts of ET, FT and float calculation. You
can also customise this excel to your needs. However, scheduling software like MS Project
and Primavera will give more accurate results and hence they are preferable.
Final Words
The importance of a critical path is undeniable in project management. Hence, the correct
implementation of the critical path method is necessary to get the correct picture of the
progress. The float consumption of near-critical activities should be well observed and
reported in advance.
The critical path should also be built with uncertainty to accommodate the minor
fluctuations in activity duration. However, the decision for the same should be made in
line with the stakeholder’s expectations.
Due to the vast concepts of the critical path method and multiple implications, only the
basic information has been covered in this article to introduce the concepts to beginners.
Also, the experts can brush up on their basics and provide feedback and expert opinions
in the comments below.
Read more about the project schedule in this beginner’s guide for the project schedule.
Get this another project schedule Gantt template for planned and actual cost, delay
analysis and dashboard.
Correctly estimate the construction labour requirement with this labour planning
template.

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