Guild of Project Controls: Compendium | Roles | Assessment | Certifications | Membership

Tips on using this forum..

(1) Explain your problem, don't simply post "This isn't working". What were you doing when you faced the problem? What have you tried to resolve - did you look for a solution using "Search" ? Has it happened just once or several times?

(2) It's also good to get feedback when a solution is found, return to the original post to explain how it was resolved so that more people can also use the results.

<b>Negative Float Problem</b>

16 replies [Last post]
Sun, 2006-05-07 07:49
Aktham ABOU ELNIAAJ
User offline. Last seen 15 years 47 weeks ago. Offline
Joined: 29 Apr 2006
Posts: 13
Groups: None
Hi every body,
I am working in a construction project, and we are updating the Primavera file each week by entering the physical progress % and the actual start date. now when we re-schedule the file by pressing F9 and entering the new data date some activities will start showing Negative Float, and the late finish will be before the early finish for example I have an activity which is in the target file (second revision) has an Actual Start of 21 March 06 and Early Finish 30 May 06 and late finish is 6 Jun 06 with a float of 6 days (OD 61 Days and Data date was 30th of March 06) now after 4 weeks of updating the schedule the early finish is 17 Jul 06 and the late finish is 31 May 06 with a negative float of -40 days. We achieved 50% as of this activity till this period.
I want to know what to be done in order to overcome this problem.
Thanks
Top

Replies

Wed, 2006-05-10 07:07
#1
Rolyn Jalea
User offline. Last seen 12 years 6 weeks ago. Offline
Joined: 2 Jun 2004
Posts: 34
Groups: None
Aktham,

I think the problem is solved? But check that milestone, maybe its a contractual milestone.
Top
Wed, 2006-05-10 05:55
#2
Emma Seaton
User offline. Last seen 10 years 43 weeks ago. Offline
Joined: 16 Sep 2005
Posts: 55
Groups: None
Well done - thought that’s what it might be. It’s so easy to miss - been there, done that and should even have the t-shirt somewhere! As I suspect do many others!
Top
Wed, 2006-05-10 05:49
#3
Aktham ABOU ELNIAAJ
User offline. Last seen 15 years 47 weeks ago. Offline
Joined: 29 Apr 2006
Posts: 13
Groups: None
hi Se,
there was one constraint in one milestone at section 2 and this was causing the whole problem.
thanks
Top
Wed, 2006-05-10 05:38
#4
Se de Leon
User offline. Last seen 2 years 49 weeks ago. Offline
Joined: 15 May 2001
Posts: 321
Groups: None
Hi Aktham,

Taking it from P3 manual:

"P3 calculates negative float only if you impose a project finish date or other constraint in the schedule".

It doesn’t make sense that you will have a negative without a constraint. If that’s the case how would P3 calculate the negative float?

Kindly check again.

Se

Top
Wed, 2006-05-10 05:09
#5
Aktham ABOU ELNIAAJ
User offline. Last seen 15 years 47 weeks ago. Offline
Joined: 29 Apr 2006
Posts: 13
Groups: None
Dear Rolyn,
I fully agree with you “Project Management Software is developed as a tool to help the construction team where they are heading and not to be played around”.
I am not trying to hide things, see our project is composed of two parts civil work and Pipeline works, with a total of 3400 activities. The overall project is divided into 14 areas in which each of them has its part of Civil and pipeline. The problem came when there was a shortage in the delivery of the pipe for section 2. What we did is we mobilized the section 2 crews to section 7 and by this we get double the progress. We made a meeting with the construction people and we studied the recourses available and we concluded that if, when pipes arrived, we shifted both crews (Section 7 and section 2 Crews) to Section 2 we can still finish before the late finish specified on the Base plan. Why is the whole project not showing a negative progress is that because section 2 is not falling under the critical path (Section 1 to Section 7 are flow lines sections while section 8 to 14 are the production sections. The production sections fall under the critical path). As of this week, the overall progress is 10.5% against 9.4 % for the plan.
I checked the constraints, and there is no constraints used in the schedule.
Top
Wed, 2006-05-10 04:38
#6
Se de Leon
User offline. Last seen 2 years 49 weeks ago. Offline
Joined: 15 May 2001
Posts: 321
Groups: None
Aktham,

Only two possibilities that I can think of why you still have negative float.

1st solution could be: Check if the activities with the negative float has a hard constraint. Check it one by one. Take out these constraints.
2nd solution: Check the must finish by date. This should be blank.

Take note that this is only a software solution. It’s possible that there is a reason why some of your activities are constrained. If you can not take out the constraints then you will always have this negative float.

I hope this helps,

Se
Top
Wed, 2006-05-10 03:48
#7
Rolyn Jalea
User offline. Last seen 12 years 6 weeks ago. Offline
Joined: 2 Jun 2004
Posts: 34
Groups: None
Aktham,

Kind advise, Project Management Softwares are developed as a tool to help the construction team where they are heading and not to be played around.
1. Why do you want to show you are ahead or on target if your programme says you are delayed by that much? Instead of trying to manipulate your programme, why can’t you just focus & find solutions on the activities that is causing such delay?
2. Delinking "Remaining Duration" & "Percent Complete" will not do any good at all. It means that you have to update twice in each single progressing activities(double work). If you want to crash the remaining duration, make sure that your construction team knows about it (and confident as well to achieve those dates)otherwise I will not recommend that.
3. I don’t know how can you say that the total float in your schedule is still zero but you have negative floats on some of your activities? check your programme again, maybe some late finish constraint is causing that negative float.
4. Sit with your construction & management team and ask what is the purpose of this programme... Is it for presentation only or as a tool to be used as a guide to improve your present work?

Regards,

Rolyn
Top
Tue, 2006-05-09 23:38
#8
Aktham ABOU ELNIAAJ
User offline. Last seen 15 years 47 weeks ago. Offline
Joined: 29 Apr 2006
Posts: 13
Groups: None
The total float in my schedule is still Zero, that means I do not have yet any delay in the finish date of the work, as far as the total physical progress, we are 1.5% ahead of the plan for the overall project. I have a delay in 15 activities, for most of them I still have some time to the Early Finish date Shown in the Base Plan, and when comparing the progress of these activities with the their relative Late plan progress from the Base plan they are ahead, except for three activities where we are well behind and we crossed the Late finish Date of the Base plan.
Before I have the "Link Remaining Duration and schedule percent complete" in the Auto-Cost Rules, checked. I removed this check in order to keep the RD different from the progress which can solve the problem for the activities where I have a delay in the progress, but still I have some time to the Early Finish of the Base plan, (that is for some activities the OD is 50 Days now after 40 days passed we done 20% only). But even this did not solve the problem and the negative float is still shown in about 40 activities even some of those having a negative float are ones which are ahead of the planned.
Do you recommend using an “Expected Finish” Constraint in order to fix my finish date and not make it moved by each time I make Scheduling!!
Top
Tue, 2006-05-09 11:52
#9
Emma Seaton
User offline. Last seen 10 years 43 weeks ago. Offline
Joined: 16 Sep 2005
Posts: 55
Groups: None
Do remember to check your activities for any start or finish constraints (look for the asterisk*) to make sure there aren’t any that shouldn’t be there. Also check your project must finish by date on the Projects, Dates, tab. If it’s not required delete it. Yes I can hear the gasps from here - but sometimes a date gets in there by accident: such as when importing from MS Projects where anything that hasn’t been linked gets a constraint date applied, including the finish date, which can screw up the float.

However, I agree with the previous posts that total float is a very useful tool in establishing when and where a programme is going into delay. I also use it as a logic check - if there’s lots of it then there may be a link missing! I have a project at the moment which has -42 days overall, and using this float path I can trace back to see what’s causing it and work out what can be done - in this case nothing as it’s a site problem caused by an unforseen obstruction in the ground. But at least I could prove it using the float path. Make them your friends not your enemies!!!!!
Top
Mon, 2006-05-08 15:19
#10
John Ferris
User offline. Last seen 5 years 33 weeks ago. Offline
Joined: 11 Nov 2005
Posts: 7
Groups: Petro-Chem / Oil & Gas
NB :

Reduce the remaining duration to that advised by the team or by calculation of OD-(ODxPCT). The basis of updating is to get the main data :

Actual Start, Actual Finish, Percentage (PCT) and Remaining Duration then schedule to next DataDate.

If the negative float then still exists the answer can be found in the advice below eg Ron’s.

Filter on all negative floats and check logic and constraints that are driving / restricting the schedule to the projects requirements.
Top
Mon, 2006-05-08 15:12
#11
Simon Peter Cordner
User offline. Last seen 13 years 27 weeks ago. Offline
Joined: 4 Apr 2006
Posts: 20
Groups: None
Ron is absolutely correct. Negative float means that its time to accelerate, resequence the work, or accept that your project is going to be finishing late.

The statement "so I guess the only way is to decrease the Original Duration for each activity" is a bad, bad, bad idea. Very bad. Forget that you ever considered it. It’s not going to help your project to make up a bunch of stuff and hope that it all works out in the end.


Re: Early and Late Finish dates in Primavera

"Early Finish" is shorthand for "Earliest Possible Finish". This is calculated based on your schedule’s progress, and is updated for unfinished activities.

"Late Finish" is shorthand for "Latest Possible On-Time Finish". This is not updated with an activity’s progress, but rather updated with the activity’s PREDECESSOR’s progress.

Here’s an example:

Activity #10, Starts at day 0, duration is 10 days, has 2 days of Total Float as a baseline.

Prior to any progress, Activity #10 has an Early Finish of day 10, due to its 10-day duration. It has a Late Finish of day 12, due to its 10-day duration and 2 days of float.

Now, imagine a progressed schedule where Activity #10 is at 50% complete at day 8. Now, Early Finish is day 13: the activity is 50% complete meaning that based on its original duration of 10 days there are 5 remaining days, and it is currently day 8, and 8 + 5 = 13. Late Finish remains at day 12, since day 12 remains the latest possible date by which your schedule will remain on-schedule. Incidentally, your Activity #10 would show -1 days of total float in this situation, since your Early Finish date is one day later than your Late Finish date.
Top
Mon, 2006-05-08 14:33
#12
N Curgus PSP
User offline. Last seen 12 years 25 weeks ago. Offline
Joined: 11 Jul 2003
Posts: 18
Groups: None
Aktham,

I would double check everything, especially the activities started and in progress, to be sure that a "bad logic" is not driving a negative float. If you are doing work out of sequence you need to reevaluate the logic and revise if it’s necessary. When you are sure that the logic is correct and then go to the steps recommended by Ron.

Good luck,
N
Top
Mon, 2006-05-08 09:21
#13
Ronald Winter
User offline. Last seen 3 years 18 weeks ago. Offline
Joined: 4 Jan 2003
Posts: 928
Groups: None
What Steve is trying to say here is that CPM is a planning tool. If your schedule shows negative float then this means that you have inserted a constraint into your schedule that is not going to be met without your changing your plan.

You have three (or more) options. 1) Admit that you are no longer going to be able to meet your planned goals by the date specified, 2) figure out some method to accelerate the work and modify your schedule to describe that acceleration, or 3) figure out some other sequencing that will shorten the overall length of the negative path and modify the logic to reflect this plan.

What I do not recommend that you do is to start shortening durations and not tell anyone. This will not solve anything other than remove the red from your plots. The purpose of a schedule is to monitor the progress of the project and predict when a problem is occurring early enough to solve the problem. As a Scheduler, you are empowered to notify your project team of the problem and to work with them to resolve it. Hiding the problem will not make it go away. Good luck!
Top
Mon, 2006-05-08 03:40
#14
Steven Oliver
User offline. Last seen 11 years 34 weeks ago. Offline
Joined: 8 Nov 2002
Posts: 313
Groups: None
"so I guess the only way is to decrease the Original Duration for each activity!!"

Try another guess.
Top
Mon, 2006-05-08 02:17
#15
Aktham ABOU ELNIAAJ
User offline. Last seen 15 years 47 weeks ago. Offline
Joined: 29 Apr 2006
Posts: 13
Groups: None
Hi,
We are using Primavera Enterprise program, and we are entering the physical % progress, so I guess the only way is to decrease the Original Duration for each activity!!
Thanks for the reply,
Top
Mon, 2006-05-08 02:11
#16
Kevin Button
User offline. Last seen 2 years 30 weeks ago. Offline
Joined: 16 Jul 2003
Posts: 112
Groups: None

If you use Duration % complete for your activities (assuming your software is Primavera Enterprise) the finish dates will be recalculated for you when you enter progress. If you are using Physical % complete you need to change the remaining duration or finish date for each activity (either one by one, or with global change). If using Units % complete I think you then need to also consider the Duration Type for the activities.

With regards to the late finish being earlier than the early finish, the float is the difference between early and late dates, so if the late date is earlier than the early date, you get negative float and you need to review your schedule.

HTH

Cheers
Top