Project Network Modeling and Activity Sequencing
Logic first, dates later
A schedule network converts the activity list and its dependencies into a graph. The dominant convention is activity-on-node (AON), also called the precedence diagram, where boxes are activities and arrows are precedence links. Activity-on-arrow (AOA) draws activities as arrows and nodes as events; it cannot represent two activities sharing the identical start and end event, so a dummy activity (zero duration, zero resources) is inserted to keep the logic unambiguous.
Four precedence types exist: finish-to-start (FS, the default in almost all real schedules), start-to-start (SS), finish-to-finish (FF), and start-to-finish (SF). A positive lag adds delay to the link; a negative lag is a lead, which overlaps work.
A valid network has one start and one finish point (or explicit milestones), no loops, no dangling open-ended activities, and every activity traceable to a WBS work package. Distinguish hard logic (physically mandatory, e.g., pour concrete before curing) from soft logic (discretionary, e.g., a preferred crew sequence) - only hard logic should drive the critical path.
Traps. Questions swap lag and lead; imply dummies consume time; suggest the Gantt chart defines logic (it only displays it); or describe sequencing by WBS order or calendar date rather than by dependency.
Rule of thumb. Draft logic with FS links only, left to right. If two activities share both the same predecessor set and the same successor set, add a dummy (AOA) or confirm they are genuinely separate (AON).
Check. A precedes B; B and C precede D. Any drawing letting C start before A finishes is wrong; valid links are A→B, B→D, C→D.