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

Meet the author:

Tal Levanon – Founder and partner in “Tal Levanon – HCP Ltd”, an expert project scheduling consultant and creator of the Hidden Critical Paths (HCP).

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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.

There are 4 different types of links:

fs, ss, ff and fs. Can be written in small letters or capital letters. 

The ‘s’ is for Start. The ‘f’ is for Finish. So, ss is ‘Start to Start’ while ff is ‘Finish to Finish’. 

The 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

fs (fs project) – 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 for fs (fs project) : after finishing writing the script for a TV episode, filming can begin.

SS - Start to Start

ss – 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

ff – 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

sf – 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😊

ff ss, 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…😊)

No one will reveal new details about your schedule?
Hmmm… 🤔

If you want to test that, try HCP-Go

FF SS - Schedule management: Rules of Scheduling

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.

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

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’.

FF SS - 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: FF vs SS links - What is 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

 FS, FF, SS, SF – 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! 😊

This Post Has 14 Comments

  1. I am still trying to understand project management tools. What does 4SS mean

    1. Hello Sheila,
      4SS means –
      The number is the ID line number. So 4 means task number 4.
      SS is Start to Start connection.

  2. I use the SF when I am building a schedule for multiple channels. Construction runs on a chronological timing, but Marketing only starts their work 20 weeks prior to Grand Opening – which I don’t know…and can change over the time of the construction – so I use the SF -100 to create the start date for them, based on the finish date for construction.

    Just because the tasks get number in the order you enter them…doesn’t mean the # matches the chronological needs of the team or project.

    1. Amy,
      The use of SF means that you think from a later date to an earlier one. As you wrote it – “20 week prior to”.
      To think from the future to the past – this is a flexible thinking – which is perfect! But unfortunately, we live only from now to the next minute, and we can’t go back.
      Schedules should describe reality – so schedules should be created from the beginning to the end. There is no room for SF connection in a realistic schedule.
      The answer to your Marketing task – is to define what really should be the trigger to start the Marketing activity. It should last for 20 weeks and then you can have the Grand Opening.

  3. You’ve made some good points there. I looked on the web for
    more info about the issue and found most individuals will go
    along with your views on this website.

  4. SF relationship is being criticised due to lack of reasonably good examples. I think aviation industry can use this relationship.

    Let ‘Boarding of plane’ be predecessor activity

    Let aerobridge door close represent Finish of the successor. This time is dictated by airport authorities since they have to coordinate many such landings and take offs.

    Let’s presume the time to board is 45 minutes .
    Here SF relationship with 45 minutes duration should be used.

    Note FS relationship is not useful because that will mean only after all the passengers board the door can close which is not the case.

    The door closure is not linked to completion of predecessor.

    ‘Hope I am clear

    1. You are right, but please notice – you are thinking from the future to the past.
      The future: Aerobridge door close
      Before that: Boarding.

      When you use flexible thinking – then you can think from the future to the past.
      But, you can live only from the past to the future.

      First – you need all the people to board, then, you can close the doors.
      So, real life is boarding – FS – closing doors.

      For calculating your project from the finish point to the start – you can write it as reversed project with SF. To perform it – you need the other way around. 😊

  5. Thanks for your prompt feedback.

  6. Hi, thanks for providing clarity on task dependencies. However , i did not understand the last part where in situation 1, you could have just given predecessor to activity ‘c’ as 1 right (activity a to ensure c doesn’t start before project date)? Why the mention of 1FS+4days?

    1. Hi Sumanth,
      The difference between writing 1 and 1FS+4 days is when ‘c’ can start.
      If it is written “1” then the duration of task c could be longer, ‘c’ can start earlier and finish 2 days after b ends.
      If it is written “1fs+4 days” this means task ‘c’ can only start 4 days from the end of task ‘a’, and if it will last more than 3 days – it will end later than 2 days from the end of ‘b’.
      Please remember, all the links, when they are translated to the math behind them (in the software), they are not “=” but they are “>=”.

  7. Is there a field in MS Project that I can open and view in the, say Entry table, that shows which tasks are FS, SF etc? I’m guessing no since multiple setups can exist for each task but thought I’d ask.

    Please let me know.
    Dan

    1. Hello Dan,

      Yes there is –

      In ‘View’ go to ‘Split View’ and Mark ‘Details’ with V. Then, take the cursor to the grey area, right click on the mouse and choose ‘Predecessors and Successors’.

      Per each task you’ll be on – you’ll see the Predecessors and Successors.

      Did that help?

      Tal.

  8. I looked this up on Google and I must say that you made my understanding of FS and SF quite simple. Also, I got one key thing into my head, “to not use SF in scheduling”. Understanding these relationships requires a lot of flexibility in thinking. Thanks, Tal

    1. Glad I could help. Thank you for sharing that with me! 😊

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