Longest Path | Primavera Scheduling

Dec

Why Make Open-Ended Activities Critical?

Categories: Constraints, Critical Path, P6 EPPM, P6 Professional, P6 Tricks, Schedule Options

Tags: Constraints, Longest Path, open ends, Primavera P6, schedule options

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by Bill Pepoon

Primavera P6 Professional is obviously a very powerful scheduling program so naturally some of its features exceed the needs of the typical project. I have consulted on projects that span as little as 35 hours to as many as 50 years. Different industries have unique requirements for their schedules as well. Primavera P6 is designed to handle a wide variety of projects. Today I would like to address my reasons for using a feature in Primavera P6 that is rarely used by the typical scheduler: Make Open-Ended Activities Critical. You will find this feature under Schedule Options (Tools > Schedule > Options).

The concept of making open-ended activities critical was introduced many years ago in Primavera P3. And for the longest time I dismissed it as a quirky feature surely not applicable to me. After all, why would I promote an activity to critical status solely because it is missing a successor? That seems akin to me declaring myself the winner of a contest that no one else entered.

Some of my colleagues back in the 1980s figured it was an en easy way to identify activities that should not be critical. Okay, that seems backwards, but the idea was that if some task showed up as critical that did not seem “right” the scheduler would investigate further. But Primavera P3 had a report similar to what we call the Schedule Log in Primavera P6 that was a more definitive (and easier) way of identifying open ends in the schedule. With this in mind, making activities that are missing a successor critical did not seem like the best approach to finding open ends.

I started my scheduling career working solely on construction projects so my viewpoints towards Primavera P3 were based on a single industry. Years later, when I began consulting on a wide-variety of projects I realized another purpose for Make Open-Ended Activities Critical. For example, let’s say I am a manufacturer trying to track progress on several production lines. I would like each production line to have its own critical path, or what Primavera P6 refers to as Longest Path. But this would entail making each production line a separate schedule.

It would be a lot easier to track progress, however, if I incorporated all of the production lines into one schedule. That way I would not have to keep opening up another schedule to track progress. And P3 only allowed four schedules to be opened simultaneously. (Some of you P3 users undoubtedly remember the master and sub-project concept from P3 which was another alternative to what I am describing). But how could each production line have its own critical path without creating separate projects?

Yep. Make Open-Ended Activities Critical. See, by not linking the production lines to each other they would all have an open end at the end of their sequence. So every production line now has critical activities. The float values are not based on the longest of all the production lines. Each production line has activities with zero Total Float. Problem solved!

The concept I just described works the same in Primavera P6 as in Primavera P3. You might be thinking that each production line could have a constraint on the final activity to create zero Total Float and then link the final activity in each production line to some final milestone. Yes, that will work too, smarty pants! It also means that additional open ends do not need to be introduced into the schedule.

Another reason for Make Open-Ended Activities Critical is relevant to projects in any industry. One of my clients currently expects his project to finish early. Owners often don’t allow the original plan to show an early completion date because it might become the basis of a claim (“I planned to finish early and you stopped me”). In this situation the owner allowed early completion. So my client inserted two finish milestones in his schedule: “Projected Finish” and “Final Completion”. The latter milestone matched the contractual finish date.

The “Projected Finish” milestone had no constraint since the date could obviously slip without any ramifications. The “Final Completion” milestone had a Finish On constraint (Mandatory Finish also works) so the date could not move at all. But as you might have guessed, this meant that only the “Final Completion” activity appeared as critical in the schedule. The earlier milestone and all of the activities linked to it (directly or indirectly) carried Total Float values based on the later milestone.

A critical path consisting of just one (the last) activity) would obviously be acceptable to no one. But the solution was quite clear to me. While my client had linked the “Projected Finish” milestone to the “Final Completion” milestone (to avoid unnecessary open ends) we need the “Projected Finish” milestone to have no successor. Then, by choosing Make Open-Ended Activities Critical in the Schedule Options the Longest Path of activities leading up to “Final Completion” all had zero Total Float. Bingo.

Below is how the schedule looked with “Projected Finish” linked to “Contractual Finish”. Not having open ends means there is no logical critical path. Also, the “Projected Finish” milestone is a non-driving predecessor to the “Contractual Finish” milestone, as evidenced by the dotted relationship line:

By deleting the relationship between “Projected Completion” and “Final Completion” and choosing to Make Open-Ended Activities Critical under Schedule Options, the activities leading up to “Projected Completion” are now critical:

Make Open-Ended Activities Critical is not always a necessary feature, but as you can see, it certainly does have a purpose.

Jun

The Schedule That Can Be Beat

Categories: Activity Relationships, logic, Primavera P6

Tags: Activity Relationships, Critical Path, Finish-to-Start, logic, Longest Path, Primavera Contractor, Primavera P6

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by Bill Pepoon

My wife and I recently participated in a group hike to see the new land acquisition by our homeowners association. We own property in a 7,000 acre resort in the Sierras and the new parcel adds several hundred more acres of wilderness that will prevent development near our resort. The problem was, our group leader had told us in no uncertain terms that the last bus back to the resort would be leaving at 5:15 pm. Yet here we were at 4:00 pm still hiking in the wrong direction and realizing we had at least an hour of strenuous hiking uphill to the staging area. So we turned around and headed back. Our group leader was clearly cutting it a little too close.

I feel the same way about scheduling. Yes, there will always be critical activities, but they are of course only critical because other activities are, well, less critical. But if the longest path of activities are themselves quite aggressive then we are only setting ourselves up for failure. So my preference is to have a schedule that can be beat.

I am not talking about sequestering float. You may have heard about strategies to hide float in a schedule. This all started several years ago because nearly all construction contracts in the U.S. state that float belongs to the project and is therefore not for the beneficial use of just one party. The contractor or the owner can use the float, with dibs belonging to whoever grabs it first.

Contractors who felt more entitled to the float began devising ways to hide some of the float by tying activities to unrelated work that starts sooner than the more obvious successors. This removes float from the schedule and makes it more likely that an owner delay to a task will also delay the project. A non-critical activity can quickly become critical because it has very little float. For this reason, the sequestering of float is often prohibited in construction contracts.

In my case, however, I am only trying to avoid being too aggressive with activities that are on the critical path and for this reason I choose the least ambitious type of relationship for the majority of my activities: Finish to Start. My goal is to make roughly 80% of the relationships in my schedule Finish to Start. This percentage can be checked using the DCMA checklist available in P6 Web. DCMA stands for the Defense Contract Management Agency. This checklist – called Check Schedule in P6 Web – can be seen below:

You set up the parameters of the schedule in this menu. Once Check Schedule is run, the results appear in a separate window, seen below:

At a glance I know whether I achieved my goal of making 80% of the activity relationships Finish to Start. Other goals include managing durations and relationship lags. Anything in green is in compliance while red indicates not in compliance with the specified parameters.

My strategy of primarily using Finish-to-Start relationships is a direct result of the early years of CPM scheduling, when Activity-on-Arrow (AOA) was the dominant scheduling technique. In those days, the arrows represented tasks, whereas the nodes (circles in most cases) were the activity identifiers. Because of this technique, each task had two nodes referred to in this order: the I-Node and the J-Node. So you would see something like Activity ID 1000-1005. This technique was also referred to as the Arrow Diagramming Method (ADM).

The Activity-on-Node (AON) method became very popular in the early 1990s and was certainly spurred along by Primavera Systems’ decision to drop Activity-on-Arrow altogether when the first Windows-based version of Primavera Project Planner (P3) was released in 1994. Ignoring this brief history of scheduling software for a moment, consider that if the arrow represented the activity, then all relationships were Finish-to-Start. The J-Node of the predecessor was also the I-Node of the successor. Leads and lags were non-existent. The predecessor(s) always had to be complete before the successor could start. This can be seen in the following Activity-on-Arrow diagram:

We used to split activities into percentages so that, say, the first 25% of drywall hanging could start as soon as 25% of the walls had been framed. This effectively became the lag that we use today with Start-to-Start and Finish-to-Finish relationships. Otherwise, it was fully expected that some work would start out-of-sequence, meaning the successor starts before the predecessor is finished.

By the same token, Finish-to-Start relationships are far more likely to generate out-of-sequence progress in today’s precedence schedules. And my response is “great!” You are beating my schedule! I have set you up for success, not failure. Now, I realize some people are bothered by out-of-sequence progress, but unless there is truly a “bust” in the logic or the result of unexpected delays I see this as a positive sign.

There is another benefit to the Finish-to-Start relationship. It is more likely our resources will not become over-allocated. Older schedulers like myself used logic to limit the number of resources required because the scheduling software was too primitive to do this. We called this type of logic “crew restraints” and I still use this technique today. The resource leveling feature in Primavera P6 is not always the best option for controlling the allocation of resources. And besides, if the schedule is not resource loaded then leveling is not an option.

My attitude often comes down to this: prove to me you can get ahead of my schedule and I will modify the logic accordingly. Until then, my logic assumes a more linear progression of the work and is therefore more forgiving. And everyone will feel better because the project end date does not keep slipping. What could be better than that?

All posts tagged Longest Path

Why Make Open-Ended Activities Critical?

The Schedule That Can Be Beat