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Dear Planners and Schedulers,
when a negative float appears in a schedule, does it affect the CPM calculations?
Thanks
Ismael
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The forward and backward pass along with durations, logic, calendars and constraints determine the dates and the float
the forward pass begins at the beginning of the project or at the data date and goes forward in time taking inot consideration of duration and logic
Forwardpass
----------------------------------------------------------->
This gives you the early start and early finish dates
The backward pass begins at the end of the project can calculates the late start and late finish dates starting with the end date of the project or the last activity and goes backward in time
<-------------------------------------------------------------------
Backwardpass
this determines the latest an activity can start and finish without delaying the project. Consider these dates as being pushed up against a wall.
the difference between the early dates and the late dates is the delta or the amount of float that you have
if your early dates are later in time than your late dates this is negative float. This means that based on were you are today or as of the data date and due to your lack of progress this is the earliest that you can start or finish something which is no beyond the latest possible date. You have gone beyond the wall.
If you WANT to see negative float to signify the delay on project completion vs contract (I don't like it, but I know many do), then you need to do either of these:
1) Create a milestone "Contractual Completion date", and give it a mandatory finish date equal to the contract date. Then link your project completion milestone to it with an FF(0) successor
2) From the Enterprise > Projects window, give the project a Must Finish By constraint date equal to the contract date.
Hi Guyz,
As we know, negative float in a programme represents delay in the project by the amount of the float value.Suppose finish date of the project is showing later than the contractual project finish date, so a negative float must be showing by that amount of days.But, in a programme, negative float is not showing any value, though project finish date shows later than the contracutal finsh date. I checked all the reasons like must finish by date, constraints, out of sequence logic. all looks ok.so how to find those negative lags?
programme is prepared in Primavera P6.
Thnx
Jace,
First:
I added the columns total float and primary constraint to see better what was happening. The project finish date was Oct-13-12. As you can see there is only one activity with constraint (Act ID 1007), that particular constraint generates negative float because it brakes the logic and the CPM calculation.
I also filter the activities where total float less than 0d and there were 18 activities.
Then:
I changed the constraint Star On to Star On or After which is less restrictive and does not affect the float. After press schedule button the negative float disappear but the project finish date is now Oct-10-12.
I sent you an email with print screens...
Regards
Ismael
Jace,
Ok, I will review it tonight.
Regards
Ismael
Ismael,
I just send u the xer file,
Jace,
send me your email address. For some reason I cant send a message to you. I get a message saying disallowed, need to send to a valid recipient.
Hilal
Jace,
sure I will send my email through the personal message
Hilal
Hi All,
I tried all ways to eliminate my -ve float, But no success, Could anybody review my schedule? I can send the file to ur email,
Hi Jace,
Please add the columns: external Early Start and external Early Finish, check if there is any date and clear them.
Hope it can help…
Best regards
Ismael
Jace,
In the Projects Window... on the Date Tab of Project details... clear the Must Finish By date...
Hi Jace
Check your calendars and resources.
Best regards
Mike Testro
Jace,
If you have checked the programme along with all the comments in this post, then I dont know how you can end up with -ve floats without looking at the programme.
I don't hav any started activities, I am just preparing the baseline, i chk all logics, lt looks ok, but i can;t identify the reason for -ve float, any help?
Hello Jace,
The problem could be the logic. If you have activity A proceeding activity B with a FS relationship and you started activity B and not finished activity A (CORRECTION), then this will create a negative float.
Regards
Hilal Hallani
I have negative float in critical path, there is no constriants and also no must finish by date in my project. Also checked relationships, it is ok, can anybody tell me wat could be the reason for negative float?
Hi Asmael,
TF is the amount of time an activitiy maybe delayed without delaying the end of the a project. TF may be -ve, so you know you are behaind your schedule by that amount and it's fine be -ve and in your current schedule NOT baseline. That what happing with me same issue. The owner said noproblem be -ve as long as in my baseline zero TF. So, no worries.
Please, let me know if you need any more info.
Thanks...
Gary,
Thanks for this very well explained answer I will keep it in mind.
To be honest, only few times a client (owner) asked me to correct the out of sequence activities, the most of them did not care or were not willing to allow changes on their approved schedule.
I also noticed some people applying a “must finish by” constraint at project level, they do that to “see better” the float generating negative values, which I think it is not necessary and maybe a not recommended practice. What do you think about this?
Regards
Negative float does not cause a problem with cpm calculations -it is a symptom of a problem with cpm calculations.
Negative float happens for one of two reasons:
1) You have out of sequence logic in your programme. -eg and activity (B) has started before it's FS predecessor (A) has finished. This causes problems with cpm because on the forward pass early start (B) is calculated as early finish (A) plus lag, but early start (A) must also equal actual start (A) for activities in progress. both cannot be true, hence cpm calcs are disrupted.
2) You have a mandatory or FNLT (finish no later than) constraint applied which the schedule cannot comply with. eg if an activty has an early finish of 2nd feb and a FNLT constriant of 1st feb. This is because the FNLT constraint forces the late finish to equal 1st feb, instead of the late finish being calculated via cpm. hence you get late finish earlier than early finish, hence negative float.
This is why you should alsways correct any out of sequence logic, and be very wary of using constriants in your schedule.
Hello Hilal,
Thanks for your answer, I always avoid to have any negative float in all my programmes and try to fix them checking the schedule log out; but to be honest, I do not really understand yet how they affect the CPM calculations and why is it a good practice to eliminate them.
I was studying the forward and backward pass but I still do not get it...
Best Regards
Hello Ismael,
yes it does. In your options setting under schedule, there is an option for "total float is less than or equal to" or you can choose "longest path". A negative float will end up being on your critical path, however you shouldnt have negative floats in your programme.
Regards
Email: PPadmin@planningplanet.com
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