You might be wondering what Start to Finish relationship is in project management.
In Start to Finish relationship or dependency, finishing event of one activity is related t0 the starting event of another activity. In simple words we can say that finish of an activity is dependent on start of another activity.
The above definition is somewhat confusing. So, I have written this article to explain the meaning of Start To Finish logical relationship with the help of examples and diagrams. You will find visual depiction of Start to Finish (SF) relationship using Project Network Diagrams and Gantt Charts in this article.
Start to Finish Relationship (Dependency) in Project Management
SF is one of the four relationships that can be drawn using Precedence Diagramming Method (PDM). The other three are:
The above relationships are used to represent and analyze project schedule. The concept of relationship originated with PDM but they can also be drawn using Time Scaled Bar Charts (popularly known as Gantt Charts).
I would suggest you to read about Project Network Diagrams before reading further.
Definition of SF Relationship
Start To Finish is a Logical Relationship in which a Successor Activity cannot finish until a Predecessor Activity has started.
In simple words we can say that, the finish event of a successor activity is dependent on the start of its predecessor activity.
You can also look at Max Wideman’s Glossary for a complete set of PDM definitions.
Project Network Diagram Representation
Let us consider two activities A and B.
- Duration of A is 3 days.
- Duration of B is 1 day.
- B has a SF relationship with A
The diagram below shows SF relationship between A and B, wherein A is the predecessor and B is the successor.
As per the diagram B’s Finish is dependent on A’s Start, which means that B can Finish (F) as soon as A Starts (S).
In the above example. the complete sequence is planned to be completed in a total of 4 days.
Gantt Chart Representation
Mathematical Representation And Formulas
Considering 0 method, we can mathematically represent the relationship between A and B as:
B(F) = A(S)
Or, if we use 1 method, we can say that:
B(F) = A(S) – 1
Start to Finish with Lead and Lag
SF relationships can be further modified by introducing Lead and Lag modifiers.
Lead example using 0 method representation
B(F) = A(S) – 1 day
Lag example using 0 method representation
B(F) = A(S) + 1 day
Start To Finish Examples
- A – Duty of Evening Guard (E), B – Duty of Morning Guard (M). M cannot Finish her/his duty till E Starts. M cannot abandon the post.
- A – Start using New Software System (N), B – Phase out Old Software System (O). It is assumed that N & O cannot be used in parallel.
Finish to Start vs Start to Finish
SF is not a very difficult concept to understand but sometimes it is confused with Finish to Start (FS) relationship. Some authors (incorrectly) claim that FS and SF are essentially same but it is not true.
FS is the most common and natural relationship but it cannot represent every sequence of activities. Although SF can be mathematically transformed to FS to achieve seemingly similar results, the mathematical transformation modifies and tampers the natural logic.
Let’s revisit the Guard example from the previous section to understand why SF is more natural than FS. Let’s try to answer the following questions.
- Is SF just a mathematical concept?
- What is the different between FS and SF?
- Where do we use SF?
- Why can’t we invert and represent SF as FS?
Finish To Start Guard Example
The above example talked about two Guards who are doing a shift duty – Evening Guard (E) and Morning Guard (M).
In a natural scenario, E can “Finish” her/his duty only “only after” M “Starts” his/her duty. The critical point here is that “E cannot Finish her/his duty before M Starts her/his duty”, otherwise E will be abandoning the post. Since E cannot abandon the post, she/he has to guard the post even if M gets delayed in starting her/his duty.
The Finish of E is logically dependent on Start of M.
Start and Finish Events
To understand the difference further, let us look at the activity events and not at activity as a whole. The two distinct events of any activity are
- Start Event (S)
- Finish Event (F)
So for these two related activities, we will have two sets of S & F relationships namely FS, SS, FF, and SF.
SF relationship only means that the Finish of Second Activity is dependent on the Start of First Activity. The Second Activity is called the Successor and the First Activity is called the Predecessor.
Over to You
PDM Relationships are logical relationships that can be represented by using mathematical equations. However, not all mathematical relationships represent the natural logic. While making a project schedule, you should ensure that all dependencies follow the natural logic and are true in the real world.
Do you think SF relationship is useful in real projects? Have you ever used it in you project(s)? Or have you seen it being used?
I would love to hear from you.
PMP Exam Formulas
I have also compiled a PMP Formulas Cheat Sheet. It contains 45 formulas and 57 abbrviations. It will help you in your exam prep. You can freely download the PMP Formulas Sheet for your studies. It is the best and most comprehensive cheat sheet based on the PMBOK Guide 6th edition.
If you are looking beyond a cheat sheet, then I would suggest you to buy detailed PMP Exam Formula Study Guide by Cornelius Fichtner. It contains detailed explanations of all the formulas along with examples and 105 practice questions.
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