5th Edition PMBOK® Guide—Chapter 6: Leads and Lags


One technique for adjusting the schedule module by reducing the duration of certain activities is adjust their leads and/or lags. This post will review what leads and lags are, describe the technique, and give a typical exam question involving leads and lags.

1. Leads and lags–definition

Definition

Leads The amount of time whereby a successor activity can be advanced with respect to a predecessor activity.
Lags The amount of time whereby a successor activity is required to be delayed with respect to a predecessor activity.

2. Leads and lags as scheduling technique

If there is a given lead or lag for an activity, it can be seen that increasing the lead of the successor activity will reduce the total duration of the two activities. Likewise, decreasing the lag of the successor activity will also reduce the total activity of the two activities. However, looking at the definitions above, and given that the lag is usually a requirement, and a lead is the amount of time that a successor activity can be advanced, it is more likely that you will be able to increase a lead rather than decrease a lag.

3. Typical exam question using leads and lags.

Activity 1 has a duration of 20 days, Activity 2 of 10 days, Activity 3 of 5 days and Activity 4 of 6 days. The relationship between activity 1 and activity 2 is FS, between activity 2 and activity 3 is FF-2 (2-day lead), and between activity 3 and 4 is FS+3 (3-day lag). What is the minimum total duration between the Milestones A and B?

  1. 36 days
  1. 37 days
  2. 39 days
  3. 42 days

This is an example of a question showing LEADS and LAGS.  You have to know the four dependencies, which are

FS  = successor activity cannot start until predecessor activity is finished 

FF  = successor activity cannot finish until predecessor activity is finished

SS = successor activity cannot start until predecessor activity has started

SF = successor activity cannot finish until predecessor activity has started

 In each of these four combinations, the first letter represents the PREDECESSOR activity, and the second letter represents the SUCCESSOR activity.   That’s a good way to remember that the PREDECESSOR activity is the one that comes first before the SUCCESSOR activity.   

FS is the most common relationship and it shows that the activities are done in SERIES, one after the other.
FF and SS are the next most common relationships and they show that the activity are done in PARALLEL (i.e., at the same time). SF is rarely used and you will not most likely not see it on the exam.

So with that background, here’s how you figure out the question.

Step 1: Activity 1 starts at 0 and ends at 20 days (you are given the duration of 20 days in the question).   When does Activity 2 start?   Well, the FS means that Activity 2 (the successor activity) starts IMMEDIATELY after Activity 1 is finished, so it starts on day 20 as well.   

Activity Starts Ends
Activity 1 0 20
Activity 2 20
Activity 3
Activity 4

 Step 2:

Activity 2 starts at 20 and ends at 30 days (you are given the duration of 10 days in the question).   When does Activity 3 start?    Well, you are given FF, so you can’t figure out directly when it starts like you could for Activity 2 which used FS.   Since you are given FF, it means you can figure out when it FINISHES.   FF – 2 days means 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.  

Activity Starts Ends
Activity 1 0 20
Activity 2 20 30
Activity 3 28
Activity 4

 Step 3:

When does Activity 4 begin?   According to the chart, it starts on FS + 3 days, meaning that it 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 is given as 6 days, the answer is 31 + 6 = 37 and the answer is therefore B.

Although not required for the answer, I have filled in the activity 3 starting date of day 23 because it ends on day 28, and has 5 days duration, meaning the start is on day 28 – 5 = 23.

Activity Starts Ends
Activity 1 0 20
Activity 2 20 30
Activity 3 23 28
Activity 4 31 37

This shows you how to use the knowledge of leads and lags to answer a typical exam question. The next post deals with the two methods of schedule compression: fast-tracking and crashing.

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