TotalFloat Calculation

stockdam

Hi can somebody clarify how the TotalFloat calculations work?

The above schedule is very simple as I was experimenting with total float.

The function I used for the column TotalFloat is "=TOTALFLOAT([Task Name]@row)".

The critical path is in red and all of the total float calculations for them is correct (zero).

TotalFloat for Task 10 is correct (40) but the others are incorrect?

Totalfloat is defined as "the number of days that a task can be delayed without impacting the Project Finish Date".

The TotalFloat for Task 2 is clearly not zero and it should be the same as Task 3; both should be 80. The calculation for Task 4 should be 130.

have I misunderstood what TotalFloat does or is there something in the schedule that is causing the errors?

Test1.xlsx

Julie Fortney

I agree - this seems incorrect. I recreated your schedule to test it out and received the same results.

With this schedule, you have 214 days to complete Tasks 2 and 3 (which have a total duration of 70 days between them) before it would impact the critical path task they precede, Task 8. The total floats of 0 and 40 make no sense to me.

@Genevieve P. Could you help us understand how this TOTALFLOAT calculation is supposed to work? Is there an error, or are we misunderstanding its function? Thanks!

stockdam

Total float is simply the amount of time that each task can be delayed before it impacts the end date of the project. For FS dependencies you start with the task that you want to calculate total float for and then follow the dependencies until you get to the earliest task that is on the critical path; you then calculate how much this task can be delayed before impacting the critical path. Or look at it another way, it's the difference between ES and LS (or LS and LF). Adding other types of dependencies (SS,FF,SF) can be tricky but I wrote some python code to correctly calculate Total Float.

As well as Total float, I would like to see Drag as an option. This shows how much you can compress a task on the Critical Path until it no longer impacts the final milestone.....ie how much time does each task on the critical path delay the project.

https://en.wikipedia.org/wiki/Critical_path_drag

It can get more complicated when you use interim milestones rather than just the final one (project end).

However this is an extremely useful aid when dealing with large, complex schedules.

I wrote some code to calculate Drag as well.

Julie Fortney - Tasks 2 and 3 have 150 working days in total before impacting the end date (in essence Task 8). Since they total 70 working days then the TotalFloat for each of them is 80 working days. You could also add another version of TotalFloat and calculate calendar days which would be useful if talking about tasks that are purely time based such as a the lead-time from a supplier. Both types have their benefits and so both options should be made available.

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