5th Edition PMBOK® Guide—Chapter 6: Precedence Diagramming Method, Leads and Lags


1. Introduction

The tools & techniques for the process 6.3 Sequence Activities are given in the following summary chart.

6.3 SEQUENCE ACTIVITES
TOOLS & TECHNIQUES
1. Precedence diagramming method (PDM) Used to construct a schedule model in which activities are represented by nodes and are linked to show the sequence in which they are performed.
2. Dependency determination Dependencies between activities are characterized by various attributes: mandatory vs. discretionary, internal vs. external
3. Leads and lags A lead is the amount of time a successor activity can be advanced with respect to a predecessor activity. A lag is the amount of time a successor activity can be delayed with respect to a predecessor activity.

In the last post, I discussed the technique of dependency determination. In this post, I will discuss the first tool, precedence diagramming method, and the third technique, leads and lags.

I will use a typical PMP exam question involving this tool and technique in order to illustrate their use.

2. Precedence Diagramming Method

Description

Note

FS Successor activity cannot start until predecessor activity has finished. Activities are in series—most common relationship.
FF Successor activity cannot finish until predecessor activity is finished. Activities are in parallel—next most common relationship.
SS Successor activity cannot start until predecessor activity has started. Activities are in parallel—next common relationship.
SF Successor activity cannot finish until predecessor activity has started. Least common relationship

Notice that in each four of these relationships, the first letter refers to the predecessor activity and the second letter refers to the successor relationship.

3. Leads and Lags

Definition

Lead Amount of time whereby a successor activity can be advanced with respect to a predecessor activity.
Lag Amount of time whereby a successor activity can be delayed with respect to a predecessor activity.

To see how leads and lags are represented in the Precedence Diagramming Method, let us take a typical exam question.

4. Exam question example

Activity 1 has a duration of 20 days, Activity 2 of 10 days, Activity 3 of 5 days and Activity 4 of 6 days. What is the minimum total duration between the Milestones A and B?

The relationship between the activities is

Predecessor Successor Relationship
Milestone A Activity 1 FS
Activity 1 Activity 2 FS
Activity 2 Activity 3 FF-2
Activity 3 Activity 4 FS+3
Activity 4 Milestone B FS

Now let’s answer the question.

Step 1. Let’s define milestone A as starting on day 0. A milestone has 0 days duration BY DEFINITION so it finishes on day 0.

Activity Starts Finishes
Milestone A 0 0
Activity 1
Activity 2
Activity 3
Activity 4

Step 2.

Activity 1 starts at 0 and it is given in the question that it has a duration of 20 days, so it finishes on day 20.

Activity Starts Finishes
Milestone A 0 0
Activity 1 0 20
Activity 2
Activity 3
Activity 4

Step 3.

When does activity 2 start? The relationship between activity 1 and activity 2 is given as “FS“, which means that Activity 2 (the successor activity) starts IMMEDIATELY after Activity 1 (the predecessor) is finished, so it starts on day 20 as well. Since it is given to have a duration of 10 days in the question, it finishes on day 20 + 10 = 30.

Activity Starts Finishes
Milestone A 0 0
Activity 1 0 20
Activity 2 20 30
Activity 3
Activity 4

Step 4.

When does Activity 3 start?    The relationship between Activity 2 and activity 3 is given as”FF-2″, so you can’t figure out directly when it starts like you could for Activity 2 which used FS.   However, since you are given the relationship as being “FF-2″, it means you can figure out that Activity 3 finishes 2 days BEFORE Activity 2 finishes.   Since Activity 2 finishes on day 30 (see beginning of paragraph for reference), Activity 3 finishes on day 30 – 2 = 28.   And since Activity 3 is given as having 5 days in duration, it must begin on day 28 – 5= 23.

By the way, the fact that the finish of activity 3 is advanced 2 days with respect to the finish of activity 2 means that this is an example of a lead.

Activity Starts Finishes
Milestone A 0 0
Activity 1 0 20
Activity 2 20 30
Activity 3 23 28
Activity 4

Step 5.

When does Activity 4 begin?   The relationship between Activity 3 and Activity 4 is given as “FS + 3“, meaning that Activity 4 starts 3 days AFTER activity 3 ends.   Since activity 3 ends on day 28, activity 4 begins on day 28 + 3 = 31.   When does activity 4 end?   Since the duration of activity 4 is given in the question as 6 days, the answer is 31 + 6 = 37 days.

Activity Starts Finishes
Milestone A 0 0
Activity 1 0 20
Activity 2 20 30
Activity 3 23 28
Activity 4 31 37

By the way, the fact that the start of activity 4 is delayed 3 days with respect to the finish of activity 3 is an example of a lag.

I hope that this example has not only illustrated the Precedence Diagramming Method, but has also demonstrated an example of how to use a lead and lag in this method.

The next week I am on a trip by car to move from Los Angeles to Chicago, so I will be blogging about other topics than project management for the next week until I get settled in the Chicago area. Thank you for your patience!

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5 Responses

  1. I am studying for the PMP certification and I would like to thank you for explaining this subject is a such understandable way. English is my third language, so sometimes is hard to figure out all the concepts. Thank you!

  2. I’d like to thank you for these impressive, no wasting time presentations you deliver here. It would be hard for someone reading this blog to come up with the usual “lost in the middle of the PMBOK forestry knowledge” which is frequently the case for PMP candidates.

    I’d like to make a small remark-question regarding the above example it seems to me that Activity 3 can not finish until activity 2 has finished (by definition of an FF relationship).
    So can we really use a lead on a FF relationship without falling into inconsistencies ?

    • Your remark Y, Jr would be correct if leads and lags jobs were not there precisely to modify relationships. Let’s say we have a A(FS-2), what does that mean ? Let’s call the successor activity B, then B cannot start until A has finished, right ? By definition of FS, but as you can see the lead modify that constrain and authorize B to start before A has finished. So there is a precedence relationship for lead you should avoid to discover during the exam (huuu, scarry) : Apply lead first and then do the computations. Now keeping these explanations reconcile you with the example’s approach as FF or FS relationship does not change anything here. In other words “Relationship give you the direction while lead gives you the possible freedom you have to drive to there”. Indeed without knowing that the next activity is B after A you would be stuck asking which one is the next? This gives us also a logical pattern: lead can even be greater than predecessor’s duration. Suppose you have A(FS-4), can A be with a duration of 3 days ? In this case B apparently starts before A. The answer is YES, as we put relationship and we apply lead to modify it so lead always give us the final situation. Here is an example: Activity A is: “do the work in 3 days” and activity B is: “send the invoice to customer and receive payment”, which takes 1 day. Now customer has called and need urgently the work and agree to pay upfront, you agree to start the job the next day but to send and receive payment today. What does that mean ? Will we change direction and make B the predecessor of A? of course not we just happened to have a lead of 4 days, A(FS-4). All in all there is no inconsistency as long as you remember that lead job is there to modify relationships.

  3. It is very helpful for understanding and making the ideas clear of the beginners especially Good job. May! Allah bless you.

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