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Late Start/Finish....Total Float - how calculated?

8 replies [Last post]
Ben Hall
User offline. Last seen 13 years 18 weeks ago. Offline
Joined: 20 Nov 2007
Posts: 18
Hi,

I hope someone can help me here...I have a schedule which shows columns so Start, Finish, Late start, Late finish, Free float, and Total float.

For example, my activity start 14/2/08, finish 6/6/08, late start 8/5/08, late finish 14/5/08, free float 0.0d, total float -16d.

How are the late start/finish and free/total floats calculated.???

where does the -16d Total float come from??

If someone can help me I would be so happy, as I am working on a massive project and being beaten down by clients over this!!!

Thanks everyone!
Regards
Ben.

Replies

Albert Balogo
User offline. Last seen 7 years 6 weeks ago. Offline
Joined: 9 Jun 2008
Posts: 44
Ben,

If you really wanted to resolve the negative float without analysing early start, early finish or whatsoever. this is the best way you can do.

1. Open file, select project overview.
2. Replace the date " Project must finish by" same the date of "early finish" under the date Project must finish by.

You have to remember negative float is from the heavy constrain you putted in your program.Just keep in your mind that the Project must finish by is the contract completion date to our client. once you filled up that date "project must finish by" its consedered heavy contrain and once your project exceeded on that date the result will be negative float.In short negative float is indacation that your project is already delayed.

Just try it and see the result

I hope it can help you

Albert
Trevor Rabey
User offline. Last seen 2 years 20 weeks ago. Offline
Joined: 29 Nov 2005
Posts: 528
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Ben, I suggested Date Constraints on 19/6.
BTW Must Finish Date is not entered at the very start.
You are not scheduling backwards from a finish date with all tasks scheduled to happen as late as possible, are you (See Project, Project Information)?
Have the Indicators column showing so you can see date constraint icons for the tasks that have them.
Ben Hall
User offline. Last seen 13 years 18 weeks ago. Offline
Joined: 20 Nov 2007
Posts: 18
Hi all,

I have figured out the problem, it was because the schedule had a "Must Finish" date of 1/1/09 and this then constrained the late start/finish dates and gave a negative float.

Once I changed the Must Finish date to the correct forcast finish date the negative float became positive and the Late S/F dates changed to suit the correct Must Finish date.

Must Finish date is entered at the very start of the project when creating a new project schedule. though because I copied the file from a similar project it took that Must Start date...all is fixed now and I am very relieved!

Thanks to Darren, and everyone else who helped me solve my problem!

Regards
Ben Hall
Most softwares calculate wrong floats if the schedule is resource constrained. Calculating resource constrained floats is much more complicated.
So don’t be surprised if you will see funny floats showed by your software after resource constrained scheduling.
Regards,
Vladimir
Darren Kosa
User offline. Last seen 4 years 42 weeks ago. Offline
Joined: 8 Feb 2008
Posts: 256
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Hi Ben,

As Trevor stated, scheduling applications use the Critical Path Method to determine ES, EF, LS, LF, Free Float and Total Float. It traditionally forms the basis of the Gantt chart, although many Planners / Project Managers never use it and it’s a skill that is being lost. Primarily because most, if not all, of the scheduling tools currently on the market are automated.

Like many things worth knowing, it may be a difficult concept to understand at first, but if you want to have an insight into the workings of your application of choice, it may be worthwhile investing a little bit of time working with Network Diagrams until you feel comfortable with how they’re calculated.

After you have had some exposure to this method it won’t take long until you grasp the theory behind the practice.

In answer to your question, Start Dates are calculated by either constraints or precedence and Finish Dates are calculated by constraints or duration.

As an example, if Task A starts on 23 Jun 08 and has a duration of 5 days it will finish on 27 Jun 08.

Late Start and Late Finish Dates are calculated in a similar fashion but they also take into account Free Float. Free Float is the amount of time a task can be delayed without affecting its successor and is the difference between the Late Finish and Early Finish.

Early Dates and Late Dates are established using forward passes (left to right along a Network) and backward passes (right to left along a Network).

Using the same example as above, but including 5 days Free Float. Task A Start and Finish Dates would remain the same but there is capacity to soak up 5 days delay without affecting the next task. Therefore the Latest Start Date is 30 Jun 08 and if you add the duration to that the Latest Finish Date would be 04 Jul 08.

If there wasn’t any Free Float then the Latest Start and Latest Finish Dates would be the same as the Start and Finish Dates.

Total Float is the amount of a Task can be delayed without affecting the Project Finish Date.

As Trevor also states in Post #4, the likely cause of your negative Total Float is due to a ‘Hard’ constraint in your schedule i.e. one that is fixed and will not move.

Regards,

Darren
Trevor Rabey
User offline. Last seen 2 years 20 weeks ago. Offline
Joined: 29 Nov 2005
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You haven’t mentioned which software you are using but it doesn’t matter.

I will assume MSP but similar story for P345666 etc.

If you are looking at MSP are you looking at the Schedule Table, or have these columns just been inserted into the Entry Table?

In MSP you have a table called the Schedule Table.
It shows the early and late starts and finishes, free float and total float.
These are always automatically calculated in the background using the good ole CPM forward and backward passes. When you are looking at the Entry Table and the Start and Finish columns there, those are the early start and early finish, provided of course that in Project Information you have all tasks scheduled from the start date and as soon as possible.
If you have negative lag (which means that even if started and finished on the early dates the project is still late) it is due to either date constraints or deadlines.
Take them off, let the project go where it wants to go, negative lag will disappear.
Hope this helps.
Ben Hall
User offline. Last seen 13 years 18 weeks ago. Offline
Joined: 20 Nov 2007
Posts: 18
thanks for the reply George,

But I really dont know much about late start/finish and total float.

My projects usually only show the Start and Finish dates.


So not sure where to go with this..
George Ridulme
User offline. Last seen 9 years 32 weeks ago. Offline
Joined: 16 Jun 2008
Posts: 21
Hi Ben,

Try this formulae

Early start + Duration - 1 = Early Finish

Late finish - duration +1 = Late Start

Late Finish - Early finish = Total Float

Hope this helps