Task Dependencies and Sequencing
Dependency types
Sequencing links activities with logical relationships. Know the four: Finish-to-Start (FS) - successor cannot start until predecessor finishes, the most common; Start-to-Start (SS) - successor cannot start until predecessor starts; Finish-to-Finish (FF) - successor cannot finish until predecessor finishes; Start-to-Finish (SF) - successor cannot finish until predecessor starts, rare. Each may carry a lead (overlap, negative lag) or lag (wait, positive lag).
Sources of dependency
Mandatory (hard, contractual or physical), discretionary (soft, preferred practice, e.g. "we always review before coding"), external (outside the project, e.g. a regulator), and internal (within the team). Discretionary links are the ones a planner can remove to compress the schedule; mandatory links generally cannot.
Traps in questions
- Reversing SS and FF. Read the sentence literally: which event gates which?
- Applying lag when the question describes lead (overlap).
- Treating a discretionary dependency as immovable; questions often ask which link can be removed to shorten the plan.
- Confusing a dependency with a constraint (a date imposed by the sponsor is a constraint, not a logical link).
Rule of thumb
Draw the arrow from the gating event to the gated event. If the successor starts before the predecessor finishes, you are using SS or a lead; if it waits, FS with lag.
Worked example: Testing (B) can begin once coding (A) has started, but only 5 days after A starts. That is A→B SS with a 5-day lag. If instead B could begin 2 days before A finishes, that is FS with a 2-day lead.