Meet the author:
Tal Levanon – Founder and partner in “Tal Levanon – HCP Ltd”, an expert project scheduling consultant and creator of the Hidden Critical Paths (HCP).
CRITICAL PATH METHOD (CPM)
The Critical Path Method (CPM) was developed in the 1950s
(For further reading: [Critical Path Method – Wikipedia])
According to this method, the earliest start dates for all tasks is calculated based on the activities’ durations and their links, known as forward scheduling.
Subsequently, the latest start dates for all activities are calculated based on the tasks’ durations and their interdependencies (links), known as backward scheduling.
The total float (=total slack) is the difference between the earliest start date and the latest start date. (It is worth noting that calculating based on the latest start dates yields the same result.)
Activities critical to CPM have a total float of 0, meaning the earliest and latest start dates for these activities are identical.
The critical path sequence according to CPM creates the critical path.
A numerical example is available here>>>
This method is crucial as it identifies activities causing the project to finish on the latest possible date.
It is essential to note that the longest path in a project is not necessarily the critical path according to CPM.
HCP METHOD (HIDDEN CRITICAL PATHS)
The Hidden Critical Paths (HCP) method, patented by Tal Levanon in the early 2000s, maps and analyzes the network of activities based on their paths rather than individual activities.
For each path, the path duration is defined as the total working days required for that specific path. Note: when counting working days for paths, a workday is counted only if at least one activity is defined for work on that day within the path.
The critical path according to HCP is the longest path in the network.
Any path shorter than the critical path receives a definition of path duration slack (DS), also known as “HCP slack” (float).
Path Duration Slack is the difference between the length of the HCP critical path and the duration of that path. For critical paths according to HCP, DS=0, while for any other critical path, DS>0.
ILLUSTRATION – THE DIFFERENCE IN APPROACH BETWEEN CPM AND HCP
In this small network, CPM would yield the result of five total slacks (total float) values:
- Total slack for Activity A – TS(A)
- Total slack for Activity B – TS(B)
- Total slack for Activity C – TS(C)
- Total slack for Activity D – TS(D)
- Total slack for Activity E – TS(E)
In contrast, HCP would identify four paths:
- A -> C -> D
- A -> C -> E
- B -> C -> D
- B -> C -> E
ARE THE TOTAL SLACK AND HCP SLACK RESULTS THE SAME?
CPM is highly sensitive to start-to-start relationships, finish-to-finish relationships, constraints, delays, and performance percentages.
HCP is indifferent to all of these factors.
In networks without the use of delays, constraints, start-to-start, finish-to-finish relationships, or performance percentages, HCP and CPM may yield similar results.
In networks employing at least one constraint, delay, start-to-start, finish-to-finish relationship, or performance percentage, CPM and HCP are likely to yield different results. In such cases, the critical path according to HCP might not necessarily be the same as the critical path according to CPM.
HCP and CPM methods do not exclude each other; instead, they complement each other.
Successfully managing a project requires equal management focus on both critical paths, CPM and HCP.
THE DIFFERENCE BETWEEN HCP AND CPM ON THE SCHEDULE
This is an illustrative example of a schedule.
Each activity in this example represents a path consists of numerous tasks.
The critical path according to CPM starts on March 1st (ID #2).
Why does it start in March and not January?
The reason: a constraint or delay. For example, one cannot start before obtaining special approval (such as a building permit), or one cannot start before a specialist is available for the job—available only in March, or any other constraint. Reality is stronger than imagination.
The moment this constraint is released on March 1st, 10 months of work are required, defining the project’s latest possible end date.
Therefore, according to CPM, activities on this path cannot be shifted in dates and have a total slack (TS) equal to zero. This is the critical path according to CPM in all existing programs.
In the HCP method, paths in the project are mapped.
Through mapping, it is revealed that there is a path different from the critical path according to CPM, starting on January 1st and lasting for 11 months (ID #3).
Since it is the longest path in the entire network, its HCP DS (Duration Slack) will be 0. This is the critical path according to HCP.
Note that this path will end earlier than the latest finish in the project, so according to CPM, it will have a total slack of 1 month.
Furthermore, in the HCP method, it is found that there is another path, also different from the critical path according to CPM, and it is longer than the critical path according to CPM—lasting 10.5 months (ID #4).
This path does not start on the earliest date in the project but a month after the project’s start on February 1st, and its finish is earlier than the project’s end date.
Therefore, the HCP DS for the hidden critical path according to HCP will be 11 days (11 months minus 10.5 months = 0.5 months = 11 days). Its total slack according to CPM will be 0.45 months.
The HCP DS for the critical path according to CPM (ID #2) will be 22 days (=1 month) since it is shorter than the longest path by a month.
On which of these three paths should project managers focus?
On all three. 💡
Project management will only be partial if the focus is on just one of the three paths.
Project managers need to be aware of all three paths and all the activities that compose them and manage them with equal managerial attention.
At a point where there is no managerial attention – there will be delays in the project, which, in turn, will lead to delaying the entire project.
The very knowledge of the three paths – the critical path according to CPM, the critical path according to HCP, and the hidden critical paths according to HCP – is the first key to the project’s success.
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This Post Has 14 Comments
I am still trying to understand project management tools. What does 4SS mean
Hello Sheila,
4SS means –
The number is the ID line number. So 4 means task number 4.
SS is Start to Start connection.
I use the SF when I am building a schedule for multiple channels. Construction runs on a chronological timing, but Marketing only starts their work 20 weeks prior to Grand Opening – which I don’t know…and can change over the time of the construction – so I use the SF -100 to create the start date for them, based on the finish date for construction.
Just because the tasks get number in the order you enter them…doesn’t mean the # matches the chronological needs of the team or project.
Amy,
The use of SF means that you think from a later date to an earlier one. As you wrote it – “20 week prior to”.
To think from the future to the past – this is a flexible thinking – which is perfect! But unfortunately, we live only from now to the next minute, and we can’t go back.
Schedules should describe reality – so schedules should be created from the beginning to the end. There is no room for SF connection in a realistic schedule.
The answer to your Marketing task – is to define what really should be the trigger to start the Marketing activity. It should last for 20 weeks and then you can have the Grand Opening.
You’ve made some good points there. I looked on the web for
more info about the issue and found most individuals will go
along with your views on this website.
SF relationship is being criticised due to lack of reasonably good examples. I think aviation industry can use this relationship.
Let ‘Boarding of plane’ be predecessor activity
Let aerobridge door close represent Finish of the successor. This time is dictated by airport authorities since they have to coordinate many such landings and take offs.
Let’s presume the time to board is 45 minutes .
Here SF relationship with 45 minutes duration should be used.
Note FS relationship is not useful because that will mean only after all the passengers board the door can close which is not the case.
The door closure is not linked to completion of predecessor.
‘Hope I am clear
You are right, but please notice – you are thinking from the future to the past.
The future: Aerobridge door close
Before that: Boarding.
When you use flexible thinking – then you can think from the future to the past.
But, you can live only from the past to the future.
First – you need all the people to board, then, you can close the doors.
So, real life is boarding – FS – closing doors.
For calculating your project from the finish point to the start – you can write it as reversed project with SF. To perform it – you need the other way around. 😊
Thanks for your prompt feedback.
Hi, thanks for providing clarity on task dependencies. However , i did not understand the last part where in situation 1, you could have just given predecessor to activity ‘c’ as 1 right (activity a to ensure c doesn’t start before project date)? Why the mention of 1FS+4days?
Hi Sumanth,
The difference between writing 1 and 1FS+4 days is when ‘c’ can start.
If it is written “1” then the duration of task c could be longer, ‘c’ can start earlier and finish 2 days after b ends.
If it is written “1fs+4 days” this means task ‘c’ can only start 4 days from the end of task ‘a’, and if it will last more than 3 days – it will end later than 2 days from the end of ‘b’.
Please remember, all the links, when they are translated to the math behind them (in the software), they are not “=” but they are “>=”.
Is there a field in MS Project that I can open and view in the, say Entry table, that shows which tasks are FS, SF etc? I’m guessing no since multiple setups can exist for each task but thought I’d ask.
Please let me know.
Dan
Hello Dan,
Yes there is –
In ‘View’ go to ‘Split View’ and Mark ‘Details’ with V. Then, take the cursor to the grey area, right click on the mouse and choose ‘Predecessors and Successors’.
Per each task you’ll be on – you’ll see the Predecessors and Successors.
Did that help?
Tal.
I looked this up on Google and I must say that you made my understanding of FS and SF quite simple. Also, I got one key thing into my head, “to not use SF in scheduling”. Understanding these relationships requires a lot of flexibility in thinking. Thanks, Tal
Glad I could help. Thank you for sharing that with me! 😊