4 types of logical relationships or dependencies
There are four types of task dependencies, and each one has its own In this relationship, task B cannot start until task A finishes. Task floats allow the project manager to see how far ahead or behind schedule a task can. precedence types are more than four. actually their are 8 types of precedence the term "min" indicates that at least "x" days between start or finish of activities. . Lets say activity A has SF relationship with B. So A has to start for B to finish. In a project network, a dependency is a link amongst a project's terminal elements. If other types of relationships are used, they shall be used sparingly and with Early critical path-derived schedules often reflected only on causal ( logical) or.
Finish to Start or FS: In finish to start relationship, start of successor activity is controlled by finish of predecessor activity. For example you can build foundations of a building after you have done excavations. Or you can test a software after you have developed it. In Finish to Start, relationship arrow originates from finish of the predecessor and finishes at start of the successor. It only means that start of successor activity is controlled by finish of predecessor activity.
In FS relationship, successor activity can start before the finish of predecessor activity if lead is applied to successor. This type of relationship is mostly used in the schedule. If you are confused about exact relationship between activities then go for FS relationship.
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Start to Start or SS: In start to start, start of successor activity is controlled by start of predecessor activity. For example when a project starts then project management activities are also started with it. Or you can start leveling of concrete when pouring of concrete is started.
It only means that start of successor activity is controlled by start of predecessor activity. In SS relationship, successor activity can start after the start of predecessor activity if lead is applied to successor. For example, if construction of walls and plaster on these walls are connected with SS relationship, then to start plaster, some portion of the walls must have to be completed. The successor has a duration of 15 days.
But the successor is also supposed to finish 5 days after its predecessor, and the predecessor has a duration of 20 days. The Finish-to-Finish relationship adds two more days to the sequence between these activities.
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We should also realize that the successor cannot start 8 days after its predecessor because the Finish-to-Finish relationship is dragging the successor by its tail. P6 has a dilemma.
In order to meet both the Start-to-Start and the Finish-to-Finish requirements, the successor would need to be two days longer. In which case both relationships would be driving.
4 Types of logical Relationships or Dependencies in a Project Schedule
The original Primavera scheduling program, P3, had an option to allow non-contiguous durations. But the displayed duration of the successor would not change. I suppose this did confuse some people because the length of the activity bar was occasionally quite exaggerated compared to the displayed duration. P6 dropped this feature, perhaps for good reasons.
We must now recognize that not all of our specified conditions can always be met. Driving and non-driving relationships can also be identified in the Activity Details at the bottom of the Activities window. Select the Predecessors, Successors or Relationships tabs.
The columns shown in these tabs can be customized to show the Driving box, which is found in the General category of columns. I always display this box. The example I provided is rather simple because there is only one predecessor activity. Imagine if there are several predecessors starting on different dates and more than one relationship type! Schedules can get very complex rather quickly because of the available relationship types.
Ah, the old days when all relationships types were Finish-to-Start, things were rather simple.
But I am still a big fan of Finish-to-Start relationships because they are the most conservative relationships. Keep in mind that as logic and durations are revised, what used to be driving may now be non-driving, or vice versa. Schedules are never static.
Out-of-sequence work can also create new situations not previously anticipated. Similarly, new activities representing scope changes or unexpected events can alter the importance of relationships. But ultimately, if the start date of a task just seems wrong, driving relationships must be analyzed first.