Schedule Dependencies (Links): FS, FF, SS, SF and all in between
© All picture rights belong to Amir Keren, author of the book "The Anthology of Knots"

Schedule Dependencies (Links): FS, FF, SS, SF and all in between

Schedule Dependencies (Links): FS, FF, SS, SF and all in between
© All picture rights belong to Amir Keren, author of the book "The Anthology of Knots"

The project’s schedule contains all of the activities (also called tasks) that must be executed and the execution logic – the links between the tasks.

 

Links are the dependencies of activities on one another, and in other terms – the priority relationships between activities.

Schedule Dependencies (Links): FS, FF, SS, SF and all in between

Schedule management: What types of links are there?

  • FS – Finish to Start.
    When Task A precedes Task B with the FS link, it means that only once Task A ends will Task B be able to start.
    As most links between activities are FS links, the letters are not written in the link title in the Predecessors and Successors columns.
    Example: after finishing writing the script for a TV episode, filming can begin.

  • SS – Start to Start.
    When Activity C precedes Activity D with the SS link, it means that only once Activity C begins will Activity D be able to begin.
    Example: only once the lecturer begins speaking, students can begin taking notes.

  • FF – Finish to Finish.
    When Task E precedes Task F with FF link, it means that only once Task E ends will Task F be able to end.
    Example: upon completing the experiment, its documentation can be completed.

  • SF – Start to Finish.
    This is the strangest link out there – if you try to think of it like the other links:
    When Activity G precedes Activity H with the SF link, it means that only once Activity G begins, Activity H can end. What?!

    This link makes no sense – if you’re looking at it on a timeline. BUT, if you flip the timeline – this link is exactly FS, just in the other direction! Meaning, if you’re planning a project from end to start – it is a logical link!

    As much as it’s important to maintain mindset flexibility – and plan a project from start to finish and vice versa, because we can only move from today to tomorrow during execution – when building a schedule: do not use this link!
    It may be possible to go back in time in films or in books, but as of today, it is not possible in real life – and should not be used in projects.

    The nitpickers would probably ask – why does this exist?
    The answer is simple. Think of F and S like 0 and 1. If you count from 0 to 3 in binary, you get: 00, 01, 10, 11. So SF also exists. It’s that simple😊

Schedule management: Lags

Every link has the possibility of a lag or delay (whatever term works for you).

For instance, in the SS link, a one-day delay between Tasks I and J will be writing like so:

Schedule Dependencies (Links): FS, FF, SS, SF and all in between

Meaning: Activity J shall begin, at least one day after Activity I began. The earliest start of J is 1 day after I. If J begins two days after I, then the condition will be met. The mathematical equation will be SJ≥SI+1day

Can a negative delay be used?
Yes, the software does allow it. BUT, a negative delay, same as a SF link, is a reverse view of the timeline! And even though this is allowed by the software – when building a schedule, you cannot use a negative delay! (Not until someone invents a time machine so we could go back in time, at which point I will update this post…😊)

Question: How can I know all of the technical rules? Isn’t there something that can check the technical aspects for me?

Answer: Today we have HCP Go, HCP’s Project addon. With one click, you can analyze your schedule file – and get a free report with your results!

Schedule management: Some say that FF links should not be used – true or false?

First rule of schedules: every task has a successor activity, excluding the final task.

We’ll mention another super-important rule: the schedule needs to describe the manner in which we are destined to execute the project, in reality. It is not a theoretical exercise in math.

 

Let’s examine the following situation, with FF (Situation 1):

Schedule Dependencies (Links): FS, FF, SS, SF and all in between

In this situation, Activity ‘c’ must end at least one day after Activity ‘b’ ends. The beginning is not dictated – and basically Activity ‘c’ can begin at the project’s start – and continue for at least one day after Activity ‘b’.

 

Is it wrong to use the FF link and should it be replaced with SS?

Let’s examine the same situation with SS (Situation 2):

Schedule Dependencies (Links): FS, FF, SS, SF and all in between

In this situation, Activity C must begin at least 4 days after B begins. And what happens to B? B’s execution may continue well after the project’s completion…

Schedule management: The conclusion?

Primavera P6 software solved the problem by allowing to link two activities with numerous links. For instance, this problem would have us link Activity B to Activity C with the SS+4 days link and with the FF+1 day link, thereby preventing Activity B from ending after the project’s completion or Activity C from starting before the project begins.

 

The Project software does not allow linking two activities with two different links. Therefore, you need to one-up them; there are two possible solutions:

 

The worst solution is saying: “do not use the SS or FF link” – because you need them, and they are valid in reality. You can forgo the SS or FF links only if you specify the activities and disassemble them to sub-activities, until no SS or FF links are necessary.
For instance, in Situation 1, if we disassemble Activity ‘b’ to Activities ‘b1’ and ‘b2’, Activity ‘c’ will only begin after ‘b1’, in parallel to ‘b2’ – we can forgo one link. Now all that remains is disassembling ‘c’ so that the link between ‘b2’ and ‘c’ is FS.

This solution is very clumsy and requires lots of work. Most people I know would be exasperated just trying to define their project in such a detailed manner (particularly if this concerns projects that contain several hundreds of thousands of activities).

 

A bad solution would be saying: “you cannot use the FF link; use the SS link instead”. Reason: Situation 2 proves that this solution is wrong.

 

A good solution would be one that defines another link – i.e., Activity B or ‘c’ will have another link that would prevent an error:

Schedule Dependencies (Links): FS, FF, SS, SF and all in between

In Situation 1, we will add a predecessor activity to Activity ‘c’ – i.e., Activity ‘a’. You can also add a 4 days lag, or not – depends on the answers to these: What makes the most sense in your project? What is realistic for your project?

 

In Situation 2, we will add a successor activity to Activity B – i.e., Activity D. You can also add a 1 day lag, or not – depends on the answers to these: What makes the most sense in your project? What is realistic for your project?

 

Think about it… You’re more than welcome to comment! 😊

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