Here we are examining the factors affecting predecessor and successor task correlation in Primavera P6. We were recently contacted by a scheduler who was confounded that entering a predecessor’s task’s actual start and finish dates earlier than planned did not affect its successor’s start date. How do we explain this mystery?
Barring any activity constraints, if the only relationship between two tasks is a finish-to-start (FS) relationship, then it is reasonable to assume that the predecessor task’s actuals, i.e., start and finish dates, should drive the start date of the successor task.
Furthermore, a fair assumption is that if the predecessor task begins and ends sooner than planned, the successor task will also commence ahead of plan. Still, in this scenario, factors may prevent the successor task’s start date from beginning sooner, even when its immediate predecessor starts and finishes ahead of schedule.
In this scenario, there appears to be no one-to-one correlation between the predecessor’s actuals and the successor’s start. It is helpful to know and understand the cause that is preventing this correlation between predecessor and successor.
This article provides a review and quick tips to examine the predecessor and successor task correlation and factors that may prevent a predecessor’s actuals from initiating the start of its successor task.
Predecessor and Successor Task Correlation
It can be disheartening and frustrating when two tasks connected by a standard FS relationship do not have a one-to-one correlation between the predecessors’ actuals and the successors’ current dates. There must be a reasonable explanation for this lack of correlation between a predecessor task and its successor.
Let’s examine the factors affecting predecessor and successor task correlation in P6 and that are the probable causes of this phenomenon.
Mystery Demonstration
In our client’s scenario, the actual start and finish dates of the predecessor were earlier than the planned dates, and the corresponding successor start and finish dates remained unchanged. Let’s investigate. My first thought was to focus on the statement that the predecessor’s actual start and finish dates were ahead of their planned dates. This could likely indicate out-of-sequence work.
Out-of-Sequence Work
There are three ways that P6 handles out-of-sequence (or out-of-plan) work for tasks that have progressed. The default in P6 Professional, and the most likely option for this schedule, is Retained Logic.
In the Retained Logic classic example, the start of the successor task violates the FS relationship with its predecessor; however, its remaining work honors that FS relationship. Thus, immediately after the out-of-sequence work or from the data date (i.e., the status date), at the end of the reporting period and proceeding forward, the relationship (in this case, FS) between the predecessor and successor is adhered to.
Refer to the article at the following link for a primer on the P6 Professional data date.
Progressing a Schedule in P6 Professional
Let us show this out-of-sequence work situation. Figure 1 displays tasks A and B connected by an FS relationship.
However, task B begins earlier than planned, and so early that it violates the FS relationship with its predecessor, task A. Task B is also in progress as of the data date. One would think that Task B continues uninterrupted immediately after the data date, but it does not; instead, it suspends maintaining the once-violated FS relationship with its predecessor, as shown in Figure 1.
This behavior is set in the schedule option “when scheduling progressed activities use,” as shown in Figure 2.
The default (and most conservative) setting is to use Retained Logic, where the remainder of the out-of-sequence and In Progress tasks are suspended before proceeding to honor their relationships with their predecessors, in our client’s case, an FS relationship. Therefore, verify that the schedule option is set to Retained Logic. For a primer on ways to schedule progressed tasks, refer to the following:
Options for Scheduling Progressed Activities in P6
In our client’s scenario, the out-of-sequence work is complete. Then, do the tasks downstream from the out-of-plan work continue from the finish of this earlier than planned work? Or do they adhere to the relationship between the out-of-sequence work and its predecessor?
In other words, can the Retained Logic setting influence tasks downstream from the out-of-plan work? Would, e.g., a FS relationship between the out-of-plan work and its predecessor hold for downstream tasks linked in series to this FS relationship, provided they fall into its purview, i.e., before the predecessor’s finish?
Refer to the blog at the following link for an article on out-of-sequence progress, Retained Logic and downstream tasks.
Out-of-Sequence Progress, Retained Logic and Downstream Tasks in P6
Figure 3 displays a schedule where the task considered (R1100) is immediately downstream from the out-of-sequence task (R1090).
This schedule utilizes Retained Logic, ensuring that the remaining durations of the out-of-sequence task (R1090) and the downstream task (R1100) adhere to the FS relationship between task R1070 and R1090, as shown in Figure 3. The out-of-sequence task R1090 is complete, so it has no remaining duration. But the downstream task R1100’s remaining duration complies (because it must) with the FS between the out-of-plan task (R1090) and its predecessor (R1070).
Well, our client’s schedule revealed that the task performed earlier than planned was not out of sequence with its predecessor.
Driving Tasks
Having eliminated out-of-sequence work, we proceeded to consider the task’s predecessors. In Figure 4, we examine all the predecessors of task W1120 to determine which predecessor drives its start date.
For a primer on driving tasks, refer to the article at the following link:
Driving and Free Float Activity Properties In Primavera P6
A quick tip from this investigation is that you can display the start and finish dates of each predecessor and successor task in the bottom Relationships tab, as shown in Figure 4. This enables efficient searching for a task’s driving predecessors.
We observe, in Figure 4, that task W1080 finishes on February 27, 2029, and task W1120 starts on February 28, 2029. Therefore, task W1080 most likely drives the start date of task W1120.
Additionally, in Figure 5, we demonstrate that you can include a “Driving” option in the columns for the Relationships tab in the bottom details.
This is another quick tip to help you search for or confirm a task’s driving predecessors promptly.
Mystery Solved
With our start and finish date columns in the Relationships tab, we quickly reviewed the dates of all predecessors. We discovered the task and relationship that most likely drives the start date of the successor task. The Driving column option would have confirmed this for us. Our client had previously reviewed this predecessor task, but not its start and finish dates.
Our client was surprised to see the predecessor and relationship that was driving the successor task in question; the relationship did not belong in the schedule. When we deleted this relationship, the start date of the task in question began as expected, resulting in a one-to-one correlation between the predecessor’s early completion and its successor’s start.
So, an unknown task relationship was the cause of our mystery. Our client was confident that this unexpected task relationship did not belong in the schedule, so the mystery was solved, and our work was done.
Summary
So, that’s a look at the factors affecting predecessor and successor task correlation in Primavera P6.
Typically, when an FS relationship connects tasks, there is a one-to-one correlation between the predecessor’s finish and the successor’s start.
However, out-of-sequence work and the Retained Logic schedule setting can interrupt this correlation. So, look for out-of-sequence progress, as this is a likely cause of uncorrelated predecessor/successor dates.
Another optimal approach is to list the start and finish columns in the bottom details’ Relationships tab. This enables the quick isolation of driving predecessor tasks.
Furthermore, you can include the Driving column to more quickly confirm which predecessor task is driving the start of the successor task in question. The key takeaways from our mystery investigation are:
(1) be aware of the effects of out-of-sequence work and the Retained Logic setting on (immediate and downstream) successor tasks, and
(2) note the quick tips to display the predecessor’s start and finish dates and Driving options in the bottom details relationship tab when investigating driving predecessors.