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Fixed work task duration calculation RRS feed

  • Question

  • Hi all,

    I am trying to figure out how Project calculates duration with Fixed works tasks. I am using a base calendar of 7.5 hours per day. I also have a calendar for other resources working on a 8 hours per day schedule. The reason I use this combination of calendars is that internal resources work on 7.5 hour days and external resources work on 8 hour days by contractual agreement.

    So the project calendar is 7.5 hours per day which is the standard corporate work day, internal resources work 7.5 hours per day and external resources work 8 hours per day.

    So I took 4 tasks with exactly the same characteristics. Fixed work, Effort driven, 10 days (or 75 hours from base calendar). I assigned 4 different resources to each task. ResourceA on a 7.5h day at 100% Capacity, ResourceF on a 8h day at 100% Capacity, ResourceD on a 7.5h day at 80% Capacity and ResourceE on a 8h day at 80% Capacity.

    From this, I get the following results.

    Gantt display:

    Task usage display :

    Task usage display timescale data :

    So I can see that for task 1 and Task 2 the Start and End dates and Duration are all the same but for task 3 and task 4 Start dates are the same, End dates are different yet Durations are the same at 12.5 days.

    Any explanation on this behavior is greatly appreciated.

    Regards,

    Jacques


    Friday, September 4, 2015 9:23 PM

Answers

  • Jaques,

    I didn't go through you whole scenario and test it, my intent is to simply point out an item or two that may have been overlooked.

    So, it looks like you have a good understanding of the singular units for the duration field, some users miss that detail.

    Something else you should check is the Project start time. Change your display setting so that the time is displayed with the date. Now check the Project Start Date under Project/Project Information. Even though you set the default start time to 8:30, there is a good possibility the Project Start Date is still at the default start time of 8:00 am. Project apparently doesn't coordinate the Project Start Time with the Default Start Time. It seems like that would be an obvious thing but I guess not.

    Hope this helps.

    John

    Saturday, September 5, 2015 5:38 PM
  • Ok, here is a fairly simple explanation, the calculation done by is in fact correct.

    What seems to be contradicting is that the 8h and the 7,5h resource have a 75 hour task with the same duration.

    Duration is the effective working time they spend on the task - so how it spreads over days doesn't really matter.

    But the "real duration" in calendar days is NOT the same - that you observed in the task usage view.

    So if the two resources put in the same effort in hour by hour - the duration measured in effective hours will always be the same for the same amount of work.

    Using different day length calendar definitions will give you some type of headache.

    Best Regards | Power2plan | http://www.power2plan.com

    Saturday, September 5, 2015 6:24 PM

All replies

  • Hi Jacques

    The percentages and the distribution of hours looks ok.

    But the duration of task 2 should be 9.38 d and duration of task 4 should be 11.72 d

    Try a recalculation. Go to the Project Ribbon and click Calculate project.

    Best Regards | Power2plan | http://www.power2plan.com

    Friday, September 4, 2015 10:05 PM
  • Hi John,

    Thanks for the reply. I did try to re-calculate but nothing changed. I must have calculated before doing the screen captures.

    Regards,

    Jacques

    Friday, September 4, 2015 10:27 PM
  • Jaques,

    You forgot to mention what setting you are using to define a "day" (File/Options/Schedule group/Hours per day). Project only has one setting for the definition of a day but since you have two different work days (i.e. 7.5h and 8h) you won't be able to use "days" in the Duration field without having to mentally figure out what it means. For example, if a "day" is defined as 7.5 h, then a resource who works 8 h is actually working 1.066 days. You are better off in this case to change the units for the Duration field to be hours, then the meaning is clear.

    Just a thought.

    John

    Friday, September 4, 2015 10:27 PM
  • Hi John,

    Here are the settings for the project as you requested:

    You can see the setup is pretty standard. This is a 7.5 hours per day schedule 37.5 hour a week. I usually plan in Days so Duration and Work are in days, but I track in hours so when I am ready to start tracking I change Work to hours.

    Here are how the 2 calendars are set up:

    I am assuming that Project uses the standard 7.5 hours Base calendar for Task Duration and Work. I am also assuming that a resource working 8 hours per day will take less time to complete the same 10 day task than a resource working 7.5 hours per day will. And this is what I am seeing in the screen captures provided earlier.

    ResourceA takes the full 10 days at 7.5h to complete the task and ResourceF also takes 10 days but has only 3 hours of work on the last day which is fine.  Whereas ResourceE is completing the task one day ahead of ResourceD but both have the same Duration of 12.5 days. I would expect ResourceE to have a duration of 11.5 days.

    I tried with minutes but the results are exactly the same :

    I was thinking that since the standard calendar starts at 8:30 AM and that ResourceE starts at 8:00 AM that Project assumed the 30 mins in the previous day thus accounting for the extra day in Duration of Task 4. But if this were the case then Task 2 should have had 11 days of Duration instead of 10.

    What could I try next?

    Regards,

    Jacques

    Saturday, September 5, 2015 3:59 PM
  • Jaques,

    I didn't go through you whole scenario and test it, my intent is to simply point out an item or two that may have been overlooked.

    So, it looks like you have a good understanding of the singular units for the duration field, some users miss that detail.

    Something else you should check is the Project start time. Change your display setting so that the time is displayed with the date. Now check the Project Start Date under Project/Project Information. Even though you set the default start time to 8:30, there is a good possibility the Project Start Date is still at the default start time of 8:00 am. Project apparently doesn't coordinate the Project Start Time with the Default Start Time. It seems like that would be an obvious thing but I guess not.

    Hope this helps.

    John

    Saturday, September 5, 2015 5:38 PM
  • Ok, here is a fairly simple explanation, the calculation done by is in fact correct.

    What seems to be contradicting is that the 8h and the 7,5h resource have a 75 hour task with the same duration.

    Duration is the effective working time they spend on the task - so how it spreads over days doesn't really matter.

    But the "real duration" in calendar days is NOT the same - that you observed in the task usage view.

    So if the two resources put in the same effort in hour by hour - the duration measured in effective hours will always be the same for the same amount of work.

    Using different day length calendar definitions will give you some type of headache.

    Best Regards | Power2plan | http://www.power2plan.com

    Saturday, September 5, 2015 6:24 PM
  • Hi John,

    OK, I did check the time and you are right, Project does not adjust the project start date-time and I had to change it to 8:30 AM. This in turn changed the first day of work to start at 8:30 for all resources and shorten the first day of work for Task 2 and Task 4 and pushed the 30 mins out to the end of the task as expected but the duration remain the same.

    Any other reason the Duration could be calculated this way?

    Thanks,

    Jacques

    Here is the resulting Task usage view :

    Saturday, September 5, 2015 6:55 PM
  • Hi John H.

    I did some testing by reversing the schedules. I set the Base calendar and Project calendar option to 8h/day and the external resources calendar to 7.5h/day. The results are slightly different. I actually got the results you mentioned in your earlier reply for Durations of 9.38 and 11.72.

    So digging a bit deeper in your explanation I was able to reproduce how Project calculates both sets of figures (i.e. based on the 7.5h day and on the 8h/day Base calendar).

    The formula would be something like this I believe : Duration = sum (Resource work hrs per day) / (units * Base calendar hrs per day) . So I get in excel the numbers below and they exactly match what is in Project :

    So OK I understand, I think, how Project calculates Duration and why the numbers are the same for the 2 calendar types when they logically should be different.

    Thanks for you help on this.

    Jacques

    Saturday, September 5, 2015 8:43 PM
  • By the way, John H. I understand why you say using 2 different calendars is a headache.

    Thanks again,

    Jacques

    Saturday, September 5, 2015 8:47 PM