After you calculate the early and late start and finish dates, you can determine the float. You will need this for the PMP Certification Exam. Float is the difference between the late and early dates.
For finish-to-start relationships, the difference between early start and late start dates will be the same as the difference between the early finish and late finish dates. However, when you have a start-to-start or a finish-to-finish relationship, you have to determine the difference between each and take the lowest number as the amount of float.
If you have a finish-to-start relationship, and the difference between your late and early start is not the same as the difference between your late and early finish, there is an error in your math!
Only Activities C, E, and F have total float. Activity F has the most, with nine days of float: It can finish up to nine days later than its early finish date and not cause the project to be late. To calculate the nine days of float, you subtract the early finish from the late finish, or the early start from the late start.
However, that float is shared between them. Any float that Activity C uses isn’t available for Activity E. If Activity C starts on Day 5 instead of Day 4, then Activity E will start on Day 9 and have only one day of float.
Activities E and F have free float. They are the last activities on a path. If they start or finish late, within their float, no other activities are affected. Even though Activity C has total float, if it starts late, then both E and F start late. But if Activity F starts late, it doesn’t cause any disruption in any other activity, as long as it finishes by Day 22.
If Activity E starts late, then F starts late, but if E finishes late by less than two days, then no other activities are impacted. Therefore, you can see that those activities with free float are the most flexible in the network. You can delay them and reallocate resources to activities that have little or no float to ensure on-time performance, assuming that resources have the right skill sets.