# Why do we need Start to Finish Relationship?

Last updated on March 8, 2016

## Start to Finish Relationship is a MathematicalPractical Concept

Start to Finish (SF) Relationship is probably one of the most confusing scheduling concept. It is not a profound concept but students still find it confounding.

During my classes, most of the students are able to understand first 3 PDM Relationships easily. There are seldom any questions on Finish to Start (FS), Start to Start (SS) and Finish to Finish (FF) Relationships. However, when it comes to  Start to Finish Relationship, there are numerous questions.

1. What is the different between FS and SF?
2. Is SF just a mathematical concept?
3. Where do we use SF?
4. Why can’t we invert and represent SF as FS?

Some of you would also have pondered over similar questions. Let us try to alleviate the confusion. Let us go back to the example that we used in the previous post on Start to Finish Relationship.

#### Start to Finish Example

The example talked about 2 Guards who are doing a shift duty. There is an Evening Guard (E) and a Morning Guard (M).

E can Finish her/his duty only when M Starts his/her duty. The vice versa is also true. The critical point is that E cannot Finish her/his duty before M Starts her/his duty. i.e E cannot abandon the post. E has to guard the post even if M is delayed in starting her/his duty. The Finish of E is logically dependent on Start of M.

In SF relationship, Finish of a Successor is dependent on Start of the Predecessor.

#### Start and Finish Events

Let us try to understand the concept in a different manner. Rather than looking at an Activity as a whole let us look at it as a set of 2 distinct events – Start Event (S) and Finish Event (F) of an Activity. So, for 2 distinct activities, we will have 2 sets of S & F events respectively. Now, let us define a logical relationship between two different activities. There could be 4 different type of logical relationships.

SF relationship only means that Finish of Second Activity is dependent on the Start of First Activity. The Second Activity is called the Successor and First Activity is called the Predecessor.

PDM Relationships are logical relationships that can be represented mathematically. However, not all mathematical relationships are logically correct. While making the Project Schedule you should ensure that all dependencies are logically true in the real world.
You can also look at some of my other articles on Precedence Diagramming Method.

Have you used SF relationship in your project? Or have you seen it being used? Or do you know of some other examples. If yes, then please leave a comment.