How do I make sure Duration calculates correctly in Project?

[bgane]

When using multiple resources, the Duration calculation appears to be off.
As an example, I will create a task for 10d (80hrs), and add two resources
(one full-time and one part-time), at 75% each (9hrs/day combined). I would
assume the Duration would calculate to 8.88d(80hrs/9hrs) and an End Date of
4/20/06 (if started on 4/10/06), but it instead calculates to 9.92d. Not
only that, but the End Date is calculated to 4/28/06 (which is 15 business
days long). What am I missing?

 

[John]

When using multiple resources, the Duration calculation appears to be off.
As an example, I will create a task for 10d (80hrs), and add two resources
(one full-time and one part-time), at 75% each (9hrs/day combined). I would
assume the Duration would calculate to 8.88d(80hrs/9hrs) and an End Date of
4/20/06 (if started on 4/10/06), but it instead calculates to 9.92d. Not
only that, but the End Date is calculated to 4/28/06 (which is 15 business
days long). What am I missing?

bgone,
There must be more to your story because something doesn't make sense.
First of all, one full-time and one part-time resource doesn't equate to
75% each (at least from my viewpoint), and/or an assignment level of 75%
each does not equate to 9 hours a day combined (I get 12 hours/day
combined).

I think we need some clarification before we can help.

John
Project MVP

 

[bgane]

I'll try to reword. I have one full-time resource (8hrs/day). That resource
can only spend 75% of their time on the task (6hrs/day). I have another
resource, which is a part-time resource (4hrs/day). That resource also can
only spend 75% of their time on the task (3hrs/day). The 6hrs + 3hrs is
where I was getting the 9hrs (or (8+4)*.75%). Hope that makes more sense.

 

[Steve House [Project MVP]]

See also my response to your other post. Are you assigning both of these
resources in one fell swoop or are assigning first the one resource to the
task and then in a second assignment operation adding the second one to it?
Also, what does the calendar look like for the part-time resource? How have
you shown he is part-time?

 

[John]

I'll try to reword. I have one full-time resource (8hrs/day). That resource
can only spend 75% of their time on the task (6hrs/day). I have another
resource, which is a part-time resource (4hrs/day). That resource also can
only spend 75% of their time on the task (3hrs/day). The 6hrs + 3hrs is
where I was getting the 9hrs (or (8+4)*.75%). Hope that makes more sense.

bgane,
Yes, that makes more sense but more questions remain (see Steve's
response). When I did a little test using two resources assigned as you
indicate on a single 10 day task, I get different results depending on
how the calendar is set up for each resource. In no case do I get any of
the values you mentioned in your original post. We need more info.

John
Project MVP

 

[bgane]

I tried it multiple different ways, and here are the results:
1) Add Resource 1 @ 75%, and then add Resource 2 @ 75%. Duration
calculates at 9.83d. The difference between the Start and End Dates is 13d.
2) Add Resource 1 and Resource 2 @ 100%, then change Resource 1 % to 75%,
and then change Resource 2 % to 75%. Duration calculates at 10d. The
difference between the Start and End Dates is 14d.
3) Add Resoucre 1, then add Resource 2, then change Resource 1 % to 75%,
and then change Resource 2 % to 75%. Duration calculates at 1017d. The
difference between the Start and End Dates is 14d.
4) Add Resource 1 @ 113% (which is really what Resource 1 and Resource 2
are at 75%). Duration calculates at 8.85d. The difference between the Start
and End Dates is 9d. This is the one that calculates correctly. I would
think the first three would calculate the same way.

To setup a part-time resource, I'm using the calendar, setting them to work
4hrs/day (8am - 12pm). Based on this, I'm assuming I can work this resource
at 75%, which would be a total of 3hrs.

Thanks

 

[Steve House [Project MVP]]

First, remember that duration is the difference between the start and end
dates MINUS any non-working time in that interval. So a task that starts on
Wed and ends the following Tue has 5 days duration, not 7, because Sat and
Sun are non-working days. Also, duration is always calculated in hours and
converted to and from days for display by the conversion factor in the
"Hours per Day" setting on the calendar options page. If that setting is
the standard 8 hours per day, for a part-timer whose calendar shows a 4-hour
workday 1 "day" of duration will require them to work 2 shifts, 2 of their
workdays. If you take a task that is 10 days duration (12 or 14 days of
elapsed time depending on whether the start date puts the total run of the
task to extend over 1 weekend or 2) and assign it to a part-timer whose
calendar shows they work 8am-12noon, the duration will remain at "10 days"
but the elapsed time shown by the Gantt chart bar and the difference between
the start and end dates will encompass 4 5-day work weeks.

Create a simple project with 1 task and 2 resources. Adust the calendar for
the second resource to show 8am-12noon as their hours of work. Split the
screen and use the bottom window to assign your resources and and adjust
your percentages and you can see exactly what is happening with each case
you ran in your experiment. Remember that your part-time resource must show
up for work TWICE, working 2 successive scheduled workdays, in order to work
for 1 8-hour "day" of duration as shown in the duration column and @ 75%
assignment the 8 hours of working time results in 6, not 8, man-hours of
work being accomplished. For you part-timer to do 30 hours of work, working
75% of a 4 hour workday, they need to come to work 10 times. Each 2 times
coming to work is counted as 1 "day" of duration because they have to come
to work twice for 8 hours of duration to pass. So the elapsed time drawn by
the Gantt chart bar will cover 2 work weeks but the duration column will
read "5 days"
--
Steve House [MVP]
MS Project Trainer & Consultant
Visit http://www.mvps.org/project/faqs.htm for the FAQs

I tried it multiple different ways, and here are the results:
1) Add Resource 1 @ 75%, and then add Resource 2 @ 75%. Duration
calculates at 9.83d. The difference between the Start and End Dates is
13d.

2) Add Resource 1 and Resource 2 @ 100%, then change Resource 1 % to 75%,
and then change Resource 2 % to 75%. Duration calculates at 10d. The
difference between the Start and End Dates is 14d.

 

[bgane]

I went back, and changed the part-time employee to have an 8hr work day, and
am just using a percentage of that resource. This seems to help. Thanks for
that idea. Now, best I can tell, using the change I made above, if I add one
resource at a time, everything seems to calculate the way I'm looking.
However, if I ever remove a resource, and then later try to add that resource
back, the calculations tend to be way off. I end up deleting the task, and
re-adding it, adding the resources in the correct order. Is that what I
should expect?

Thanks

 

[Steve House [Project MVP]]

IMHO, you were doing the part time employee correctly before, having their
calendar showing the actual hours they are going to physically be on the
property and available to work. There's a difference in the task times for
4 hours of work achieved by a half time resource with 4 working hours a day
who works the morning hours working 100% versus the same 4 hours of work
achieved by a person with an 8 hour calendar working at 50%. In the first
case, the task starts at 8am and ends at 12noon. In the second case the
task starts at 8am and ends at 5pm. Big difference if you have to tell the
person doing the next task in line when they should be prepared to start
work on it!

Remember the difference between effort driven and non-efort driven behaviour
deals with how Project calculates revised durations when you add or remove
bodies from a task. In effort driven the total man-hours of work get
pro-rated between the resources. With non-effort driven they get
replicated. I strongly suggest handling this sort of problem with the split
screen, using the bottom window to adjust the resources assignments. That
way you have a "dashboard" with all of the controls at your finger tips and
can see exactly what is happening in the task and you can dial-in the exact
amounts of work, etc, you want each resource to do.


--
Steve House [MVP]
MS Project Trainer & Consultant
Visit http://www.mvps.org/project/faqs.htm for the FAQs