Calculating Float in a PND

Float, or slack, is the amount of time a delayed task can postpone the project's completion. Technically, there are three different types of float:

• Free float This is the total time a single activity can be delayed without affecting the early start of any successor activities.

• Total float This is the total time an activity can be delayed without affecting project completion.

• Project float This is the total time the project can be delayed without passing the customer-expected completion date.

Most project management software will automatically calculate float. On the CAPM or PMP exam, however, candidates will be expected to calculate float manually. Don't worry—it's not too tough. The following describes the process.

Examine the PND and find the critical path. The critical path is typically the path with the longest duration and will always have zero float. The critical path is technically found once you complete the forward and backward passes. Start with the forward pass. After the backward pass, you can identify the critical and near-critical paths, as well as float.

VIDEO How to calculate float.

1. The early start (ES) and early finish (EF) dates are calculated first by completing the forward pass. The ES of the first task is one. The EF for the first task is its ES, plus the task duration, minus one. Don't let the "minus one value" throw you. If Task A is scheduled to last three days, it would only take three days to complete the work, right? The ES is one, the duration is three, and the EF is three, because the activity would finish within three days, not four days. The following illustration shows the start of the forward pass.

Activity

2. The ES of the next task(s) will be the EF for the previous activity, plus one. In other words, if Task A finishes on day eight, Task B will begin on day nine.

3. The EF for the next task(s) equals its ES plus the task duration, minus one. Sound familiar?

4. Now each task moves forward with the forward pass. Use caution when there are predecessor activities; the EF with the largest value is carried forward. The

4. Now each task moves forward with the forward pass. Use caution when there are predecessor activities; the EF with the largest value is carried forward. The

5. After the forward pass is completed, the backward pass starts at the end of the PND. The backward pass is concerned with the late finish (LF) and the late start (LS) of each activity. The LF for the last activity in the PND equals its EF value. The LS is calculated by subtracting the duration of the activity from its LF and then adding one. The one is added to accommodate the full day's work; it's just the opposite of subtracting the one day in the forward pass. Here's a tip: The last activity is on the critical path, so its LS will equal its ES.

6. The next predecessor activity's LF equals the LS of the successor activity, minus one. In other words, if Task Z has an LS of 107, Task Y will have an LF of 106. The following illustration shows the process of the backward pass.

7. The LS is again calculated by subtracting the task's duration from the task's LF and then adding one. The following shows the completed backward pass.

8. To officially calculate float, the LS is subtracted from the ES and the LF is subtracted from the EF. Recall the total float is the amount of time a task can be delayed without affecting the project completion date. The next illustration shows the completed PND with the float exposed.

Activities with float

Activities with float

Project Management Made Easy

Project Management Made Easy

What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.

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