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Dear all,
Please advice if number of float paths considerbaly more, what we can tel about that schedule?
Hi Vladimir,
I do not fully understand Spider's resource-scheduling engine, but I have the feeling that "leveling" is too simple - or perhaps too crude - a term for what it does. Resource-optimizing scheduling might be more appropriate.
I'm sorry that I do not see you on the list for Construction CPM Conference this year. I could use a refresher.
Rgds, tmb
Hi Tom,
I also don't like the term leveling and prefer resource-constrained scheduling.
But I am curious why you would not call leveling Spider Project resource scheduling engine?
By the way Spider Project calculates resource constrained total floats and resource critical path (since 1993).
Regards,
Vladimir
Hi Bensot,
This is a nice write-up, and I agree with you that the “Critical Path” for any particular project may not be obvious or easy to define. On reading your post, I’m afraid you left out the single most important factor distinguishing the two methods: variable activity calendars. I’d also highlight the impacts of multiple late constraints and Resource Leveling
1. Variable activity calendars;
2. Multiple Late Constraints
3. Heuristic Resource Leveling
Activity Calendars (forgive the length – copied and pasted from another post I wrote)
The fundamental reason for the existence of the LP method is that it manages to define a single “Critical Path” to project completion while avoiding the pitfalls of activity calendars in the traditional method.
As Zoltan has already described, the traditional CPM definition of Critical Path is based on the logical path through the schedule – from Project Start to Project Finish – with the lowest Total Float value. This works fine for simple projects and is the default method for P6. With variable task calendars, however, the analysis becomes complicated because each activity's total float is computed according to its own calendar, and two activities that are on the driving path for project completion can have different total float values simply because they have different calendars.
For a simple example, begin with two Critical tasks in a simple project with a standard 5-day weekly calendar: Task A is a 4-day-duration task running from Tuesday morning to Friday evening. Task B is a 4-day-duration task starting the following Monday morning. Task A is the only predecessor for Task B and is in fact its driving predecessor. Both tasks have Total Float = Zero, and the traditional Critical Path definition (TF=0) works fine. Now, while keeping everything else the same, we apply a 7-day weekly calendar to Task A and recalculate. None of the (early) dates change, but the float values for Task A and its predecessors do.
The Free Float of Task A is now 2 days - i.e. Task A really could slip two days (into the weekend, according to its own calendar) without delaying Task B. Task B's Total Float remains at Zero, but Task A's Total Float = 2 days. As a result, the traditional Critical Path report is truncated at Task B, excluding Task A and its chain of driving predecessors. You might then choose to increase the "critical" float threshold to capture these TF=2d activities in your report, but now you run the risk of including additional activities that are truly 2 days from driving anything. In any case, the resulting filter/report does not show a single Critical Path extending from the Project Start to the Project Completion.
The Longest Path analysis ignores Total Float and concludes that an activity is "Critical" if it is on the driving path for project completion. This path can be determined by working backward from the project completion and analyzing relationship floats to determine the driving predecessor(s) of each activity in turn, according to the calendar of the activity being examined. In our simple example, the Longest Path method will properly define the Critical Path – including Task A and its driving predecessors – by correctly identifying the driving relationship between Task A and Task B.
These complications arose almost immediately with the introduction of variable task calendars, and Primavera offered the Longest Path method as an alternate (and in my opinion, more correct) means to define the Critical Path. (Note that in the absence of variable task calendars or late constraints, then the traditional method and Longest Path method will define the critical path equally.)
Late Constraints
When one or more late constraints (e.g. “Must Start/Finish On or Before”) are imposed, P6 will compute the total float for each activity according to its controlling (most “urgent”) downstream constraint. Consequently, the last task in the project (i.e. the end of the longest path) may have zero float, while negative float exists due to several constrained milestones earlier in the project. Ignoring activity calendars (i.e. if all activities are on a single calendar), then that group of activities with the lowest total float may be taken as the “Critical Path” for the project, even though it is in fact the driving path to the “most constrained” milestone. Moreover, since logical paths through the project typically intersect, the driving paths for all other constrained milestones (or the project finish) are unlikely to share the same value for total float. Finally, using total float alone, there is no way to guarantee that the “most constrained” milestone doesn’t change from update to update. “Critical Path for the project” is hardly meaningful under such circumstances. Especially when the effects of variable activity calendars are added, Total Float becomes unreliable as an indicator of “Critical Path” or any other logical path in the schedule.
Since P6’s “Longest Path” criteria ignores total float, it is generally immune to these effects, so it still shows the driving path to completion of the project. If project success is tied to an activity that is NOT the last one in the schedule, however – for example, a “Start Production” milestone that is 6 months before final completion – then the Longest Path may not in fact be the “Critical Path.” I have found Multiple Float Path analysis worthwhile for that condition. Khuong Do (the originator of this thread) wrote a nice introduction: https://doduykhuong.wordpress.com/2016/01/12/create-critical-path-for-any-activity/. I had a more detailed article some time ago: http://wp.me/p6CCB4-L.
Resource Leveling
As Rafael notes, Primavera has indicated that the Longest-Path criterion is not compatible with resource leveling. Although I have not done any testing to verify in P6, I would be equally suspicious of any logic-based schedule analysis (including total float criterion) in the presence of heuristic (i.e. rule-based), resource leveling. These leveling algorithms impose a delay on some activities based on resource-usage priorities – not logical sequence relationships – and it is not clear (to me) how these priorities are captured and reflected in P6’s activity dates and float calculations. (Rafael includes graphics from Spider Project, which seems to offer a pretty sophisticated resource-scheduling engine. I would not call it “leveling”.)
I have done some testing of resource-leveled schedules in Microsoft Project and found that the indicated critical path (by either total float or logic tracing approaches) did not adequately capture the impact of resource leveling/constraints. http://wp.me/p6CCB4-v
Critical Path Vs Longest Path
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.
Source of Document : Web.
Bensot.
OOPS the Longest Path Got broken.
http://www.mosaicprojects.com.au/WhitePapers/WP1043_Critical_Path.pdf
When you have multiple paths with 0 or negative float each represents a critical path, it is not true there must be a single critical path, not even in ancient CPM theories. It is not true it is from the start to the end.
When you have a portfolio schedule representing multiple jobs, there is no such thing as a single critical path.
CPM scheduling have gone a long way and it is long overdue we discard old theories that do not fit on modern models.
It might be that an activity is not critical by traditional Start/Finish Float but one of its assignments might be, this assignment belongs to the critical path. By virtue of one of the independent team assignments Activity 1 is critical, it contributes to the schedule duration, if you eliminate the activity or make its duration equal to zero the schedule duration will be reduced by 6 days of activity DRAG.
The critical path is the path of with the least amount of Total Float through the project
The Longest Path is the path through a project network from start to finish where the Total Duration is longer than any other path.
The Longest Path is a Critical Path, but not all Critical Paths are the Longest. For example I can have a path that has -5 days of negative float and I can have another path that has -15 days of float.
For example in this case since the total float of both of theses paths are less zero or less then they are categorized as critical but the path that has -15 days the MOST critical and is the longest path.
if you are looking to see what is delaying your project look at the longets path
If you are resource leveling your schedule and have date constraints P6 Longest Path will braeak and will not be reliable.
https://docs.oracle.com/cd/E17462_01/Web_Access/Help/en/Projects/Tasks/p...
The longest path is broken when activities are no longer driven by relationships; that is, when activity dates are driven by constraints or resource leveling.
Longest path calculation includes interproject relationships. Therefore, activities designated as on the longest path may change depending on whether you schedule a project alone or with its related projects. If a project has interproject relationships and you schedule it alone, the interproject relationships are treated as scheduling constraints.
Dear Mr. Khuong,
There are some instances in project that all the activities in the project are delayed but we need to find the activities that is driving the completion date of the project. one of our projects is delayed for more than one year and ofcourse we rely on longest path to control the project in line. If I define critical path based on total float <=0 in this project, I will end up with all the activities in the schedule highlighted with red in colour. In reality, these all activities may not be critical comparing the longest path which drives the project. If there are more than 1 longest paths/ near to longest path, then it would be easier to define these paths based on total float values. (Eg: critical path>> Total float <=-345).
Email: PPadmin@planningplanet.com
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