Critical path method: total float, free float and near-critical
The critical path is the longest continuous sequence of activities in the schedule, and it sets the earliest finish date. Total float is how long an activity can slip before the finish date moves; free float is how long it can slip before its immediate successor moves.
The critical path
GAO defines the critical path as the longest continuous sequence of activities in a schedule. It sets the earliest completion date. When the network has no date constraints, critical activities have zero total float, so a one-week delay on one of them delays the finish by one week unless it is recovered.
Scheduling software marks as critical every activity with zero or negative total float. The critical path is not constant: it changes as activities are delayed, finish early or run out of sequence. Date constraints, multiple calendars, out-of-sequence progress and resource leveling can all distort float, so GAO prefers the longest path when the two disagree.
Total float and free float
Total float is the time an activity can be delayed or extended before the program finish date is affected. It can be positive, zero or negative. Free float is the part of total float available before the activity's delay affects its immediate successor.
An activity can have total float but no free float. GAO's example: an activity with 2 days of total float and zero free float can slip 2 days without moving the finish date, but its successor will start 2 days late. If that successor belongs to another team, that team's start has to be replanned.
Negative total float is time that must be recovered to avoid missing a constrained date. It appears when a constraint date is earlier than the date the network logic allows. GAO says a significant delay calls for a plan that examines recovery options; only if the negative float cannot be mitigated should the milestone forecast a slip.
Near-critical activities
Activities whose total float is within a narrow range of the critical path's float are near-critical: a short delay can make them critical. The threshold depends on the program. A brief schedule might use 5 days; a program lasting years might use 2 or 3 months. GAO says management should monitor both critical and near-critical paths.
Where the exam catches people
Believing an activity with total float can slip without affecting anyone: with no free float, its successor starts late even if the finish date holds.
Believing only zero-float activities need attention: near-critical activities can become critical after a short delay.
Believing negative float means you rebaseline at once: first develop options to recover the time, and forecast a slip only if it cannot be recovered.
Believing the critical path stays fixed once set: it shifts as work progresses, and constraints can make many activities show zero or negative float.