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6 - PROJECT TIME MANAGEMENT

On any network path, the schedule flexibility is measured by the amount of time that a schedule activity can
be delayed or extended from its early start date without delaying the project finish date or violating a schedule
constraint, and is termed “total float.” A CPM critical path is normally characterized by zero total float on the
critical path. As implemented with PDM sequencing, critical paths may have positive, zero, or negative total
float depending on constraints applied. Any activity on the critical path is called a critical path activity. Positive
total float is caused when the backward pass is calculated from a schedule constraint that is later than the
early finish date that has been calculated during forward pass calculation. Negative total float is caused when a
constraint on the late dates is violated by duration and logic. Schedule networks may have multiple near-critical
paths. Many software packages allow the user to define the parameters used to determine the critical path(s).
Adjustments to activity durations (if more resources or less scope can be arranged), logical relationships (if the 6
relationships were discretionary to begin with), leads and lags, or other schedule constraints may be necessary
to produce network paths with a zero or positive total float. Once the total float for a network path has been
calculated, then the free float—the amount of time that a schedule activity can be delayed without delaying the
early start date of any successor or violating a schedule constraint—can also be determined. For example the
free float for Activity B, in Figure 6-18, is 5 days.

6 5 10
B
Path A–B–D = 25
11 5 15
1 5 5 16 15 30
Start A D Finish
1 0 5 16 0 30
6 10 15
Path A–C–D = 30
C (Critical Path)
6 0 15
KEY Activity Early Early
Node Start Duration Finish
Activity Name
Late Total Late
Start Float Finish
Critical Path Link
Non-Critical Path Link

Figure 6-18. Example of critical Path Method

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©2013 Project Management Institute. A Guide to the Project Management Body of Knowledge (PMBOK Guide) – Fifth Edition 177

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