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How does the activity calendar affect the total float

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daniel ishmael
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I am trying to analyse the project longest path and the float is varying from 0 to 2. Will the type of calendar affect the project Total Float?


Rafael Davila
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Suppose we have a job to do consisting of two activities A and B, both in parallel with a duration of 5 days.

You will be in charge of Activity A, and you have no right for vacations during a year. I will be scheduled to perform activity B but in two days will leave on vacations for 30 days.

I will be driving the duration of the job to be finished in 2 (hardly working)+ 30 (on vacations in paradise)+ 3 days (again hardly working), my vacations will open float for your work. You will be able to delay your work by 30 days without further delaying the finish of the work.

Yes calendars are very relevant but unfortunately software developers do not provide us with some visual representation on the Gantt Chart. Most are still hooked on the FS no lag relationship of ancient CPM theory, as if constraints do not matter, and one of the most forgotten constraints are calendars.

Remember calendar work must be synchronized among the different resource and activity calendars, you need a view to the combination of calendars to determine a strategy of possible changes in calendars or resource assignments.

I use Spider Project and we can schedule for partial assignments to be either fixed or variable, and this add to the difficulty on forecasting available time from the user perspective even when the software can handle the issue with ease.

To add to this difficulty remember that lag also have calendars of their own. For example two activities working during the regular work week might have a FS link with a lag of several days using consecutive calendar days. I also use special lag calendars to set up specific hour of the day or specific week day an activity is to start as a special calendar for the activity wont do it.

Yes constraints make it difficult to understand but you cannot model real life without these and many other constraints.

Best Regards,


Roland Tannous
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To better understand why this really happen, think about how is your CPM software calculating total float as?

TF = LF - EF  ( LF = Latest Finish - Earliest Finish)


TF = LS - ES (LS = Latest Start - Early Start)

TF=LF - EF recommendable.


Now Draw a sample network made of 3 activities . A, B & C.

Link those three activities with FS relations . (For the sake of simplicity). FS = Finish-to-Start

A with a calendar of 5 days per week.

B with a calendar of 7 days per week.

C with a calendar of 5 days per week.


Try when and if Activity B ends on a Friday. Look at the float values. Why are they so?

what is the TF of Actiivty B in this case considering that Activity C is on a 5 day per week calendar and most probably starts on Monday.


Best Regards,


User offline. Last seen 2 years 43 weeks ago. Offline
Joined: 21 Aug 2006
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Hi Daniel, Indeed the total of an activity is calculated using the cakendar which is affected to the task. For that reason, when you re working with a multi-calendar schedule the longuest path can have a different total float on each one o its task! Hope it answers, Rgds, Maxime