What is a Precedence Diagram Method (Network Diagram)?
The precedence diagram method (PDM) draws the schedule as a network: activities are boxes, dependencies are arrows. It's the map the critical path is calculated on. Four dependency types exist — finish-to-start (the workhorse), start-to-start, finish-to-finish, and the rare start-to-finish.
Dependencies also have a nature: mandatory (physics — concrete cures before you build on it), discretionary (preference — we like to finish design first), external (the permit office), and internal. Exam scenarios love asking which type a given dependency is, because only discretionary ones can be relaxed when you fast track.
Worked example
In a software release network: "write API" → finish-to-start → "integrate frontend"; "write test cases" runs start-to-start with development. When the deadline moves up, the PM looks for discretionary links to overlap — the mandatory ones (can't test code that doesn't exist) are untouchable.