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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