Phasing and Schedule Flexibility
Schedule flexibility is another factor to consider when determining whether to sequence or to parallel activities. In a while, we will discuss the topic of “slack” or “float” – activities that can take longer than planned without affecting the overall project schedule.

Look at the diagram for phasing. If any of the three activities is delayed or takes longer than planned, the entire project schedule is extended – everything in a phased project is on the critical path. Now look at the diagram for paralleling. In this case, activities A and/or C might take longer than planned without affecting the overall project schedule, depending on how these activities are linked to activity B.

Slack or float give you schedule flexibility, and you cannot have slack or float without paralleling.