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.

what is the physical meaning of a slack

12 replies [Last post]
Ahmed Ewis
User offline. Last seen 8 years 11 weeks ago. Offline
Joined: 29 Jan 2007
Posts: 28
Groups: None
does anybody there guys knows what is the physical meanings of a slack
does negative slack something good ?? ( me i have a feelings which say its not !!!)
does too much slack is something good ?? (me i think its a weakness in planning like taking so much factor of safety!!!)

can any body give a professional answer for me?
and if i have too much positive slack can how can i optimize it?

I will wait for some answers please

Replies

Rafael Davila
User offline. Last seen 15 hours 22 min ago. Offline
Joined: 1 Mar 2004
Posts: 5228

Amhed,

  • If applying % to total duration it is to be expected to get some fractional days as remaining duration. Say 50% duration on a total of 3 days equal 1.5 days.
  • Instead of using %duration you can use whole days for remaining duration, but this will have the effect of delaying some unwanted activities.
  • No matter what, in practice some activities will be delayed on the field for reasons such as it does not make sense to start the activity if it is not going to be finished the same day, another reason will be after certain hour it does not make sense to start certain activity. On the other hand for many activities it makes perfect sense to start as soon as possible even if it is not at first hour of the work day.
  • This is one of the many reasons why we should schedule considering management of buffer time.
  • I favor using hours as the planning unit and not be so concerned about the precise hour the activity is scheduled to start because you never know ahead of time the exact hour, better manage on the optimistic schedule than a delayed schedule. 
  • If required to round remaining durations I would round up greater than 1 day remaining durations to the current integer if fractional is </= 0.75 to the next integer if not, instead of using 0.50 as the threshold. If remaining duration is less than 0.75 days 0 is not an option but fraction of a day might.
  • I do not believe there are simple rules except not rounding at all.
  • Best Regards,
Rafael
Ahmed Ewis
User offline. Last seen 8 years 11 weeks ago. Offline
Joined: 29 Jan 2007
Posts: 28
Groups: None

Hi Guys

I have a problem and I can not solve it

I have a master time in which I saved a baseline and I added a status date then after this I stated on tracking gantt to add the precentage of the works then I made the reschedule (simply pressed the button) so the completed activities remained on the base line and the future tasks moved automatically after the status date

Then I observed that the duration started to have fraction I really dont like this as the client will not understand what does it mean 42,8 days!!!

is there any way to remove this to make it 43 or 42 days? I tried to go to option schedule and to choose task as fixed duration and to uncheck the new task are effort driven but is still didnt work

any advice I need help to solve this

 

Regards

Ahmed Ewis
User offline. Last seen 8 years 11 weeks ago. Offline
Joined: 29 Jan 2007
Posts: 28
Groups: None

Hi Guys

I have a problem and I can not solve it

I have a master time in which I saved a baseline and I added a status date then after this I stated on tracking gantt to add the precentage of the works then I made the reschedule (simply pressed the button) so the completed activities remained on the base line and the future tasks moved automatically after the status date

Then I observed that the duration started to have fraction I really dont like this as the client will not understand what does it mean 42,8 days!!!

is there any way to remove this to make it 43 or 42 days? I tried to go to option schedule and to choose task as fixed duration and to uncheck the new task are effort driven but is still didnt work

any advice I need help to solve this

 

Regards

Rafael Davila
User offline. Last seen 15 hours 22 min ago. Offline
Joined: 1 Mar 2004
Posts: 5228
Ahmed,

Close to optimum solution on a schedule, where many things happen and most frequently you don’t finish on originally planned date is acceptable to me, go for it.

Yes given no contract time limit, looking for an optimum or near optimum duration makes sense.

Essentially I believe you will have to look for the activities duration that will yield minimum cost for a given duration and record its cost, repeat the procedure for different durations and the solution that gives you minimum this is it. The challenge is in finding the solution set for each duration as some activities duration will be at the left of the individual minimum while others at the right of their individual minimum, the minimum cost if alone.

Finding the individual activity time-cost curve although time consuming is a piece of cake as compared to manually finding the network combined time-cost curve.

Best regards,
Rafael
Ahmed Ewis
User offline. Last seen 8 years 11 weeks ago. Offline
Joined: 29 Jan 2007
Posts: 28
Groups: None
dear Rafael

I think i can prove a way of optimization i will work on it and i will feed it back in fact its simple you can introduce in MS project the resources and the activities for 2 project versions one with big slack and another with small slack value and see the cost
if we transfer it in money value it will be enough to understand or to have a feeling of how we can make it anyway i will check and i will give to all a feed back maybe u can do it too


Rafael Davila
User offline. Last seen 15 hours 22 min ago. Offline
Joined: 1 Mar 2004
Posts: 5228
Perhaps you are looking for time-cost trade-off functionality:

http://www.netmba.com/operations/project/time-cost/

This optimization technique is seldom used as it is common for jobs to be contracted under limited duration and because of the many experienced delays is considered of not much use.

I can recall software called Artemis that in the late 70’s provided such functionality; I am not sure if the time-cost trade-off functionality is still available. Combining time-cost trade-off with limited resource constraining can be a daunting task or even conflicting, I do not believe they considered analyzing both issues at the same time. Eventually the issue on limited resource allocation became of major relevance.

http://www.aisc.com/index.php?option=com_content&view=article&id=47&Item...

Best regards,
Rafael
Ahmed Ewis
User offline. Last seen 8 years 11 weeks ago. Offline
Joined: 29 Jan 2007
Posts: 28
Groups: None
Dear guys
I don’t want a fight here its hard thing to understand even prof planners cant understand
the efficiency of your planning of a project is depending and i have feeling of this on the slack time you have so if i made so much slack values this mean im not a good planner cause i allow so much time free time available for every task to have a delay like
if i have an installations to be done in a ceiling of a building and from my site experience i know that such installations needs only 1 week and i give to it 2 weeks so the closing of the ceiling which depend on my finishing of such installations will lose one week cause i over estimate my time which is not wise of course u can cheat any client with it so i know and i feel i need to be realistic
but in the same time with factor of safety in every tasks or if im more specialist with tasks that i know it can cause delay like machine handling to the site
the machine transport can cause delay or machine erection can make more delay the weather when i left such heavy machine on the roof for example can cause delay
the question is if i know this and i have a feeling about it how can i optimize my plan after i check the slack time and after i see it on the Gantt chart
if anyway body think about it will find it not an easy task in fact its the final touch to make ur plan optimized
cause also so much free time available will reflect on ur resources values and also your project estimated budget
duration means time and time means money salaries and wages so anybody here made such optimization???

who can tell that my plan is better than zaher or samir or mike or x or y plan ???
who can tell if my or ur plan are more efficient ???
BR
Samer Zawaydeh
User offline. Last seen 5 years 4 weeks ago. Offline
Joined: 3 Aug 2008
Posts: 1664
Dear Rafael,

The question of "physical meaning of a slack" is very smart question. Did you know that I did a search and found nothing.

We need to define a description from our experiences. Which is basically how much time you have before the delay of the activity is going to delay the project.

With kind regards,

Samer
Rafael Davila
User offline. Last seen 15 hours 22 min ago. Offline
Joined: 1 Mar 2004
Posts: 5228
From the PMI Practice Standard for Scheduling.

Article 3.3.2.5
Float values are calculated by comparing the early and late dates as follows:
-Total float is calculated by subtracting the early finish date from the late finish date (start dates can be used instead).
-Free float is calculated by subtracting the early finish date of the activity from the earliest start date of the closest of its successors. Free float is never negative.

Article 4.2
Definition for Total Float:
Represents the amount of time an activity can delay its finish without impacting the late finish of the project. It is computed as the difference between the late and early dates of the activity, calculated from the forward and backward passes respectively. As

Definition for Free Float:
Represents the amount of time an activity can delay its early finish without impacting any successor activities early start. It is the difference between an activity’s early finish date and the earliest start date of the closest of its successors.

Just burn the PMI document as it is inconsistent, at times it defines float using start dates while at times it defines float using finish dates. This is not necessarily true after progress is reported or when using non continuous PDM (split activities).

What blow out our mind is that some PMI members are proponents of the Non Continuous PDM Method, a method that allows for automatic activity spitting, this is wrong in the case you want to keep the activity continuous, this hides that individual segments do have early and late dates and floats of their own. This is also mostly implemented across the board and not at the activity level, in both cases a wrong approach, shall not be automated. It is geared to please the lazy scheduler. Simply separate activities into two or more individual activities and allow the software to display all values.

Look at P3, it allows you to make it your own definition, either the difference of start dates or the difference of finish dates while Microsoft Project plays the roulette by allowing it to be the largest of the two. Well to kick the activity laying down on the floor some software allows the use of resources to be discontinuous along the activity duration, this fools the resource leveling algorithm and again is geared to please the lazy scheduler. Resource usage also have float, let the software math see it, create separate activities when needed to properly model your job.

Best regards,
Rafael
Ahmed Ewis
User offline. Last seen 8 years 11 weeks ago. Offline
Joined: 29 Jan 2007
Posts: 28
Groups: None
thank you guys my project is not so big in fact i have three projects for electromechanical installations
one is a hotel which will take around 12 months
and another is 2 floors building
and another is a restaurant
but i wanted to see how this affect me
but in case i have too much slack value how can i modify this ok a main task is having so long time not talking about subtasks
Samer Zawaydeh
User offline. Last seen 5 years 4 weeks ago. Offline
Joined: 3 Aug 2008
Posts: 1664
Hi Ahmed,

The physical meaning is that how much "spare time" you have before you start delaying the activity and the overall project.

Slack or float is good because it takes care of the uncertainities and the management reserve for it.

The Schedule is as good as your plan. Physically, you have to assess (this is a repetitive task that you can do on a daily basis) how the delay of the ongoing activities will affect the overall project duration.

Question: what is the size of your project please.

With kind regards,

Samer
Mike Testro
User offline. Last seen 1 week 4 days ago. Offline
Joined: 14 Dec 2005
Posts: 4418
Hi Ahmed

Slack is also called float - it represents the spare time available before you have to start a task.

Zero slack means the task is critical to the completion date.

Negative slack is not good because it will have been generated by an artificial constraint that stops the programme operating correctly.

Excessive slack is caused by sloppy logic linkage - every task must have at least 1 outgoing link to another task.

Extensive slack is also caused by use of SS FF Links combined with lead lags - I always advocate use of FS links only.

Best regards

Mike Testro