What is a Forward Pass & Backward Pass?
The forward pass walks the network left to right computing each activity's earliest start and finish; the backward pass walks right to left computing the latest dates that don't delay the project. The gap between them is float — and wherever float is zero, you've found the critical path.
Exam mechanics: forward pass takes the largest predecessor finish (you can't start before all inputs arrive); backward pass takes the smallest successor start. Total float = late start − early start.
Formula
Total float = LS − ES = LF − EF
Worked example
In a plant-shutdown network, the forward pass says valve replacement can finish by day 9 at the earliest; the backward pass says it must finish by day 9 at the latest. Float: zero — it's critical, and the maintenance chief now knows that crew's coffee breaks are the project's coffee breaks. The parallel painting activity shows ES day 3, LS day 8: five days of float, five days of scheduling freedom.