SensiblePM101 – 002 | Dependencies Predecessors Successors

Hello welcome to the SensiblePM 101. This is Mark Phillipy and I am the Sensible Project Manager. Today for SensiblePM 101, I wanted to talk a little about task dependencies, and specifically about predecessors and successors and their relationships between the tasks. Now this is a really important concept to understand when you are building your schedule and as you are putting together your WBS.

Now, what is a predecessor and a successor. A predecessor is a task whose start and finish date determines the start and finish date of the successor, or the follow on task. The successor is a task whose start or finish date is driven by its predecessor, or the previous task. So there’s a relationship between the predecessor and the successor. Quite simply if you think of it from a simple point of view, in general, and this is not always true, but in general, a predecessor comes first, the successor comes second. And now the reason why I say that it isn’t always true that its one right after the other is because there is also a relationship between the predecessor and the successor. Theres actually two things that we take in to consideration when we talk about the task and the relationships between those tasks or those dependencies. One of them is the type of relationship between two tasks, and secondly the lag between two tasks.

Now lets talk about type first. There are four different type. Those four types are first of all, what is refered to as a finish start, or in Microsoft Project a FS. Now a finish start, really what happens with that is that the successor task cannot start until the predecessor task finishes, and that is the default in Microsoft Project. What happens in this case the predecessor task will complete, well let’s say for instance the predecessor task is fourty hours. That fourty hours has to be complete before the next task or the successor task can start. That is one type, so a finish start. So a start start or an SS in Microsoft Project is where a successor cannot start until the predecessor starts. In other words they essentially start at the same time. So, this is defining a relationship where the successor and the predecessor start at the same time. The next type is a finish finish and it is very similar to a start start in that a finish finish, or a FF in Microsoft Project, is the successor cannot finish until the precessessor task finishes. So in other words, both of them finish at the same time. And the final type is a start finish. This is I think is kind of rarely used, but it is there. So in this case a start finish or SF in Microsoft Project, is where the successor task cannot finish until the predecessor task starts. So again I think this is a fairly rare case, but there are times when you might not want to finish a successor until the predecessor starts. I generally do not use that just because it doesn’t make as much sense to me as using a finish start in a finish start relationship. So those are the different types.

The other thing that we consider when we are talking about the relationship of tasks, is the lag that you have between two tasks. Now the lag is really that. It is the time between the successor and the predecessor. Take into consideration those four different types we talked about, that lag is relative to those relationships. Now, for instance, on a finish start if a predecessor finishes and you want to start the successor five days after that then you would have a lag of five days for your lag. So if the predecessor is again a fourty hour task, so that’s going to be a one week task, five days later you would start the successor. Now with the other types it might be the case where you are going to start, if you have a start start relationship, then you might start the successor five days after the precessesor starts. You can also have a negative lag, so that for instance if I have five day lag that I am going to have between the predecessor and the successor, I could say that I want to start that successor five days before the end of the processor finish. So that’s how the lag works and that’s the types of relationships you start to build on a project. Understanding this is very critical as you start to build your project schedule and those tasks on your WBS. Its critical because if you could get this relationship and understand what those relationships are between the tasks and you can get those clearly defined, as execution occurs on the project and you have changes in the durations or other things that might effect the schedule, that begins to show wheither your schedule is coming in on time, or if you are late and how you might need to make adjustments to be successful.

OK, well I hope that conversations about task dependencies, predecessors, and successors and relations, help you as you begin to understand those relationships in a project schedule. Now if you want to have more of these practical project management tips, come visit us at www.sensiblepm.com. I would love to have you visit there often and I will continue to leave these practical tips on sensiblepm.com under sensiblepm 101. You also can come to iTunes and subscribe to the podcast. If you wish to contact me, you may send me an email to podcast@sensiblepm.com. You can find me on Twitter @sensiblepm. You also can find me on LinkedIn. My last name is spelt Phillipy. Now remember, The Sensible Project Manager always looks for the practical way to manage a project to success.