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PMP lesson · Schedule · Schedule · lesson 1 of 4 · about 7 minutes

Dependencies, leads and lags

Predictive · hardware in hybrid projectsFree preview
Goal: After this lesson you can name the four logical relationships, classify dependencies as mandatory, discretionary, external or internal, and use leads and lags correctly.

1The situation

“Why can't they start tomorrow?”

The customer visits the robot cell site and sees the electricians waiting. “Why are they not wiring yet?”

Because the control panel is not installed yet, and you cannot wire a panel that is not there. But the programmers could start offline programming now, even though the plan says after design approval: that order was only a preference.

Knowing which sequences are physical rules and which are choices is the key to building, and later compressing, a schedule.

2Four logical relationships

Every link in a schedule network is one of four types:

TypeRuleExample
FS finish-to-startB cannot start until A finishesInstall panel → wire panel
SS start-to-startB cannot start until A startsStart pouring concrete → start levelling
FF finish-to-finishB cannot finish until A finishesFinish programming → finish validation
SF start-to-finishB cannot finish until A starts (rare)New shift starts → old shift can finish
FS is the most common. If a question says “cannot finish until…”, think FF.

3The four relationships

FS A B B starts after A finishes (most common)SS A B B starts after A startsFF A B B finishes after A finishesSF A B B finishes after A starts (rare)
The arrow shows which end of A controls which end of B.

4How it looks on the exam

Exam-style question 1. A software team proposes starting system testing three days before the final module is complete, testing the finished modules first. Which scheduling technique are they proposing?
A. Adding a lag of three days
B. Applying a lead of three days
C. Converting the relationship to start-to-finish
D. Resource levelling
Show the answer and the decode
Answer: B.
In simple English
Testing will start three days earlier than the normal sequence.
What is the question really asking?
The name of this adjustment.
Key words / trigger
“three days before the final module is complete”
PMP logic
Starting a successor before its predecessor finishes is a lead (negative lag).
Why the wrong answer looks attractive
Lead and lag sound similar; a lag would make testing start later, not earlier.

5Remember this

Your memory card

  • FS (most common) · SS · FF (“cannot finish until”) · SF (rare)
  • Mandatory = physics/law/contract · Discretionary = preference (can fast-track)
  • External = outside the project · Internal = inside
  • Lag = wait · Lead = start early
The full lesson in the PMCLEAR app also has:
  • the rest of the lesson (Four kinds of dependency, Leads and lags)
  • a worked example
  • the common traps (wrong vs right)
  • 2 exam-style questions with the decode
  • a 3-question quick check
Open the full lesson → Create a free account
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