When it comes to your project’s Critical Path, there are few topics more important to your client. The expectation is that YOU, dear scheduler, are the expert. So YOU had better know your project’s Critical Path inside and out.
But wait! ……Is it the Critical Path you should be reporting on, or the Longest Path?
The terms Critical Path and Longest Path are often used interchangeably when discussing your project’s super-high-priority sequence of work to deliver on time. It’s confusing to many. And there are some great debates about the differences.
The good news is that there IS an important distinction between your project’s Critical Path and the Longest Path and we’ll explain it to you here.
What is a project’s Critical Path?
Well, it depends.
“It is the path of 0 Total Float through the project.” – popular definition.
Well, the above definition for Critical Path IS true but only in under a very strict set of circumstances. When IS it true?
A. No Project Deadline.
When we set a deadline on our project (ie: a Must Finish By date in Primavera P6), it results in the appearance of negative Total Float if you are late, and positive float if you are early. Thus, the path of 0-float activities vanishes.
B. No Constraints.
Constraints affect an activity’s Total Float value. Constrained activities can show negative Total Float and can drive negative float to their successors or predecessors. Applying contraints throughout a project will affect what activities are Critical. And so, again the path of 0-total float through the project eludes you.
C. No Actuals.
A project with actuals may not show a path of 0-float either. The actuals and the order of execution can affect the activities’ Total Float as well.
So it seems that the age-old “path of 0 Total Float” definition doesn’t fit in our complex project environments.
And if you care to do some digging online, you’ll find that our attempts to clear up the concept of “Critical Path” has led to numerous new ways to define the term “Critical” for a project. Search and you are likely to run into terms like:
- Criticality
- Critical Chain
- Near-Critical
- Critical Risk
So which activities are critical? It all boils down to this – it depends.
What will be your project’s Critical Path depends on how you end up defining the term “Critical” and specifying what activities are “Critical” in your eyes. You may have activities that you deem “Critical” regardless of what the software says their Total Float values are. You may have activities that have positive Total Float but are Critical in the client’s eyes. You may want to track a specific path of activities through a project. This is called real-world project controls.
“Critical Path is a path through your project which has activities YOU consider critical.”
What is a project’s Longest Path?
There’s more clarity to be found here. The longest path is THE LONGEST path – there’s no debating the term “longest”.
The Longest Path is the path through a project network from start to finish where the Total Duration is longer than any other path.
It makes sense to focus on a project’s Longest Path as it is definitely the path where deviations from plan will affect the project’s finish date.
Is there overlap between Longest Path and Critical Path?
Let me say it this way – the Longest Path is a Critical Path, but not all Critical Paths are the Longest.
Because we can alter Critical Path through constraints, actuals, deadlines and other mechanisms, often we end up with a path that has more Critical activities than would be on the Longest Path.
If you start with a baseline schedule that has no actuals, no constraints and no deadline, and you perform some CPM scheduling on it, the path of 0 Total Float will be the Longest Path. If you add up the durations of all activities on that 0-float path, it will have the longest Total Duration.
If you calculate the Longest Path for a project that does have actuals, constraints, a deadline, etc, you will come up with a path of activities that are “important”, need I say “small-c critical” to the timely completion of the project.
Which one should I use?
I would strongly suggest you get to know your project’s Longest Path and monitor it. What the client wants to see might be something else, but the timely completion of Longest Path activities leads to your project finishing on time.
Most software packages let you track both Critical Path and Longest Path. Track both.
Lastly, I would advocate educating yourself on how the software that you use defines Critical Path and what sorts of things (constraints, relationships, lag, etc.) will affect an activity’s Total Float.
What Do You Think?
The Critical Path vs Longest Path debate is long from over. What are your thoughts about which one to use and why? Let us know in the comment section.
No constraints, no actuals and the longest path wins hands down as the critical path. This quality of a schedule network like this will substantially help with Primavera Risk as well.
Your Comments
Depends on the type of project, how it is set up and what you are measuring against. Eg Critical path, 0 float drives the expected turnover date. Where as the longest path: Scaffold dismantle or Post Turnaround activities do not affect the set measurment.
Projects like this do not fair well in Primavera Risk unless you set up to ignore longest path activities.
Why when i keep my Project as Longest path and shows those Activities when have TF=0 in Longest path and when i update my schedule then if more activities has got negative float for this time the longest path doesn’t show the others Activities on longest path without of those Activities which i have set in my Baseline schedule but it show in Critical path.
@Tim – I think it’s interesting how we sometime have to tailor the structure of our projects so the software works properly. So is the software helping build good schedules or are we trying to help the software work?
Within a project or an integrated project made up of 2 or more sub-projects, we may have some important milestones to meet. For example, a sub-project has to be completed by certain time within the integrated-project period. One of the ways is to impose a “must finish” date constraint for the sub-project and in this case “critical path” is used, which will make the activities along this critical path to have 0 or negative float. So, plus the longest path of the integrated project, the schedule will show 2 critical paths.
@SF
It’s good practice to be aware of both. When you set a deadline on an activity via a constraint, it will change critical activities and thus critical path. But your longest path is a true path that affects the project’s finish date. I recommend monitoring both.
Hi Mike,
I am a planning engineer for deputed for an High Rise Hotel Project. I really liked the discussion which clarified some my concept regarding the Critical path.
I have a question, our client here wants us to monitor all the near critical path running beside our Present critical path. Could you suggest me an easier method in Primavera to Find out all the ” Near crititcal APth”, as this i have to do it every week for the progress presentation. These process i am using is time consuming.
Hi Ashish,
You’ll want to use a feature called Multiple Float Paths. I don’t have any tutorials on it at this time, but you’ll find options by clicking the Options button on the scheduling screen, and looking at the Advanced Tab. Turn on Calculate Multiple Float Paths, pick an intermediate milestone to end with. Reschedule, then look at columns “Float Path” and “Float Path Order”. All activities on a Path will have the same Float Path number (eg: 2) and you can sort them by Float Path Order.
To find a near-critical path, look for activities with small Total Float values.
Hopefully this will help you get started, but let me know how you make out.
M
My opinion is that the longest path defines the shortest time the project can be accomplished while the zero float highlights activities that need to be prioritized to make the on-time achievement of the calculated finish date possible.
I am of the “zero float” school of thought, and i feel that many times, we do not do a thorough job in articulating all the internal and extraneous factors about our project while making our project schedule therefore the issue of zero float-critical path definition seems to be flawed. There is so much politricking, laxity and maneuvering in many projects from the planning stage up to the execution completion stage that make subject of criticality confusing to many people.
As project professionals, there are standards that must be adhered to. Here we have the zero float and the longest path methods setting the standard for critical path definition and we need to strife to achieve compliance instead of looking for an easier way that further increase our laxity and lousiness.
Thanks for the comment Shola. I would agree with the confusion on this and this discussion is helping to illuminate those standards. What sorts of factors don’t usually get articulated while building schedules, in your opinion?
you can also use filter for finding near furture critical activities
define in filter creteria total float 5, 10 , 2 days as you want
Well its bit difficult specially when critical path is popular in organization & People expect that software will give real picture…Let me give you an example..My Primavera was showing TF= -30 days for an activity, when i go to engineering manager then he says do not focus this activity , & do tell this to client & do not report…is there any way to hide this in primavera….
@ Michael – How we have to act in such a situation.
In fact You are more than word can explain. I really validate every analysis of your training. Thanks for been there.
Our pleasure!
I find the difference between the longest and critical path still confusing. My beginner questions:
1) Can there be more than one longest path within one simple project ?
2) Can there be more than one critical path in a simple project ?
3) Does the longest path always start with the first planned activity in the schedule and ends at the last planned activitiy ?
4) How does the algorithm for the longest path work ? Does it also look at total or free float of an activity ?
5) The ‘path’ part is still unclear for me. For my understanding a critical path is always a connected path of activities which satisfy a certain criterion ( for example TF ,= 0 )
How do we cope with 10 activities chained after each other, all with Total Float = 0, except for 2 activities in the middle due to some constraint or a different calendar. Are all 10 activities then considered to be on the critical path, because they are connected or will the 2 activities with a positive TF value be excluded ? When they are excluded, we don’t have a path anymore, right ?
My definition:
CP is TF<=0 (imho)
LP is the longest path to complete the project on time. Any deviation will effect the finish date of the project. This will be the CP prior to project start. You can have multiple LPs in a programme but this is rare. If TF0 then there is no CP but still a LP.
Hi ,
One of my project schedule is totally completed. All actual dates are fed in. Now, i want to find out what was the longest path in actual scenario. How to find it?
Rgds.
i am reviewing one schedule and schedule is showing negative float for many activites without any contstaint in schedule on any of activity. how it is possible, any one can explain?
You might have a must finish by date set on the entire project.
thank you very much for your response. one more question: where i can find that entire project is set for must finish date? project end milestone do not any constraint.
I think more discussion needs to be centered around the calculations when using different task/resource calendars. I believe “longest path” calculations consider this while the critical path loses it’s identity when using multiple calendars (i.e. tasks on the “critical path” don’t share the same float value). Someone please fell free to help a brother out. Good discussion, btw.
Insightful discussion.
I agree with Major, if the activities in your schedule have multiple calendar assignments (5-D WW,
7-D WW) or you are implementing the use of resource calendars, the “longest path” is the better choice due to critical activities having different total float values.
Brian/Major, agree that multiple calendars can mess up CP when using Total Float – I have yet to prove it with an example, but many experts have rumbled about this issue. In case you’re interested in finding the truest critical path, there’s another post I’ve written about it here: Primavera P6’s Longest Path Conundrum (and How To Fix It) https://www.planacademy.com/primavera-p6-longest-path-conundrum/
What is thev difference between the time length of any path and the critical path?
I’m not quite sure that there’s a difference between the longest path and the critical path because by definition the longest path to completion is the critical path with the lowest Total Float whether the Total Float is negative, zero or positive. Completed activities default to zero Total and Free Float so whether they are on the critical path or not is irrelevant since the critical path is from the most recent status forward to the completion. Multiple calendars used in a project will create anomalies in the Total Float calculation but they will be easily identifiable and most of the time bracketed by the “real” Total Float number.
The comment that items may seem critical to me but not the customer is a bit confusing because the Project Scheduler doesn’t usually “own” the schedule. If a task is determined to be critical and not on the critical path then it seems that the logic is wrong and needs to be adjusted.
Sonali had an interesting question about how to find the critical path from a completed project. It seems the best way to find what was the critical path from a completed project would be to simply remove the Actuals and move the completion Must Finish on Constraint back two weeks and follow the lowest Total Float.
Jim, the longest path is actually calculated by totaling up durations of all the paths through the project. The longest one is the Longest Path.
Hi Michael,
In a schedule with actuals, the longest path is not starting from the baseline start date.
Is it due to out of sequence activities? Please reply here and mail to my email ID also.
Thanks a lot.
Hi Khaja, when a schedule has actuals, those completed activities are no longer Critical nor can they be on the Critical Path. This is a basic rule of CPM scheduling – only the remaining work will be on the Critical / Longest path.
For highlighting nearby critical activity we can use project setting and define total float less than or equal to like say 10days and after that we can monitor it.
Hello Michael. thanks for the topic. more powers to your elbow. please i like to know if it is possible to determine the next critical path in a failing project. i know that if a project is progressing as planned then the critical path should not change. but in most cases projects do progress out of sequence and this give rise to multiple critical paths. i do experience this whenever i am updating my schedule. if i close the first activity on a path another path will come up with another activity. how can i determine this other paths in advance.
You can identify near-critical paths by analyzing total float of your activities. If an activity isn’t on the CP it might be near-critical if it has a small total float. You can use the Multiple Float Paths tool in P6 to do an in-depth analysis (too complex to explain here) but that’s no guarantee. As you progress, your CP will split and diverge, often to near-critical activities, but elsewhere as well. I recommend monitoring activities with small float, near-critical, to predict where it may go.