Slack and Float When constructing a network diagram, there may be activities that can be delayed without impacting the project schedule. These activities are said to contain float or slack. The terms ‘slack’ and ‘float’ are synonymous. For the exam, you need to know the definitions of the three types of slack/float that you can encounter on a project: Free: the time an activity can be delayed without delaying the Early Start of the successor
Total: the time an activity can be delayed without delaying the project end date
Project: the time the project can be delayed without delaying another project
A number of activities in a network diagram occur in parallel. This is because different resources with different skill sets are able to perform their jobs independently. For example, the jobs of setting up a database, installing CAT 5 cable, developing a user interface, and installing a phone system all require different skill sets. It is quite possible to execute these activities in parallel as dependencies between them may be minimal. Therefore, it is quite possible that while one path in the network diagram takes 10 weeks, a parallel path may only take 5 weeks. This is where we address the idea of slack or float in a project timeline. Slack/float definitions: • • • •
ES - early start, the earliest time in the network schedule an activity can begin LS - late start, the latest time in the network schedule an activity can begin without impacting the late finish (LF) EF - early finish, the earliest time in the network schedule an activity can end LF - late finish, the latest on the network schedule an activity can end without impacting the start of a successor activity
Float or Slack can be computed with the formulas shown below. Slack/float = LS – ES
or
LF – EF
©℗ 2009-2013 Richard Perrin – Evolutionten: Project Time Management
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