Duration Calculation in MSP

[Ritesh]

Hi

I have a very basic question.

My Project has 8 hours per day. (Hours Per Day Setting)

Resource A works for 9 hours per day and Resource B works for 8 hours
per day.(I Set up the Resource Calendar)

I created a fixed units task (say Task 1) with 65 hours of Work and
assigned both A &B to this task at 50%. Now Project Calculates the
duration for this task as 9 days with work assigned to both A & B as
32.5 hours.

How does Project Arrive at the duration of 9 days, is my question?.

As Work = Duration * Units, going by this logic:

For A => 9 days * 9 hours per day * 50% = 40.5 hours of Work
For B => 9 days * 8 hours per day * 50% = 36 hours of Work
Total = 76.5 hours, where as the work that I entered is 65 hours.

How does Project actually do the calculations?

 

[John]

Hi

I have a very basic question.

My Project has 8 hours per day. (Hours Per Day Setting)

Resource A works for 9 hours per day and Resource B works for 8 hours
per day.(I Set up the Resource Calendar)

I created a fixed units task (say Task 1) with 65 hours of Work and
assigned both A &B to this task at 50%. Now Project Calculates the
duration for this task as 9 days with work assigned to both A & B as
32.5 hours.

How does Project Arrive at the duration of 9 days, is my question?.

As Work = Duration * Units, going by this logic:

For A => 9 days * 9 hours per day * 50% = 40.5 hours of Work
For B => 9 days * 8 hours per day * 50% = 36 hours of Work
Total = 76.5 hours, where as the work that I entered is 65 hours.

How does Project actually do the calculations?

Ritesh,
Well here's how it all works out.

First, because you are assigning both resources when the task is
initially created, Project will allocate the total task work hours
equally between the two resources (i.e. each resource has 65/2 = 32.5
hrs.)

Next, in the absence of work contouring, Project will linearly spread
each resource's hours with resource A working 4.5 hours per day and
resource B working 4 hours per day. At that rate, resource A works 7
days at 4.5 hours per day which sums up to 31.5 hours. Then on the 8th
day, he only needs to work 1 hour to make up his total of 32.5 hours.
Similarly, resource B works 8 days at 4 hours per day and one half hour
on the 9th day for his total of 32.5 hours.

Third, duration is the difference in working time between the task start
and task finish. In your scenario the easiest way to validate the task
duration is to look at the task Start and Finish fields (include the
time). In my test of your scenario, the task start is 5/22/08 at 8:00 am
and the task finish is 6/3/08 9:00 am. The working time difference is 72
hours and given a setting of 8 hrs/day for duration calculations, that
is 9 days exactly.

Probably the best way to see the above is via the Task Usage view, with
the timescale set to show days and hours.

Hope this helps.
John
Project MVP

 

[Jim Aksel]

switch to the task usage view to see how the hours are spread. What I see is
the program is honoring the 50% units (4.5hr/day for A, 4 hours/day for B)
up until the last two days.
On the last two days, Resouce A works 1 hour on day 8 and no hours on day 9.
Resource B works 4 hours on day 8 (per 50% allocation) and then 0.5 hours
day 9.

It appears reasonable the program should be working resource A 1.5 hours on
day 8 instead of 1 hour. This would let the task complete in 8 days and
certainly does not violate any availability issues for resource A.

Perhaps someone else will post who has more in depth knowledge of the
assignment algorithm. My SWAG tells me it takes the 65 hours and divides it
equally amongst the number of resources present, then runs out those hours
based on the assignment units (50%) which determines the number of days that
resource will need to run.

I did an experiment with 60 hours and three resources. Sure enough,
everyone gets 20 hours. So it appears the first thing project does is divide
the work equally among the resources present, then uses Units% and that
fractional portion of the work to determine the duration of a resource
assignment.

--
If this post was helpful, please consider rating it.

Jim

Visit http://project.mvps.org/ for FAQs and more information
about Microsoft Project

 

[Ritesh]

Ritesh,
Well here's how it all works out.

First, because you are assigning both resources when the task is
initially created, Project will allocate the total task work hours
equally between the two resources (i.e. each resource has 65/2 = 32.5
hrs.)

Next, in the absence of work contouring, Project will linearly spread
each resource's hours with resource A working 4.5 hours per day and
resource B working 4 hours per day. At that rate, resource A works 7
days at 4.5 hours per day which sums up to 31.5 hours. Then on the 8th
day, he only needs to work 1 hour to make up his total of 32.5 hours.
Similarly, resource B works 8 days at 4 hours per day and one half hour
on the 9th day for his total of 32.5 hours.

Third, duration is the difference in working time between the task start
and task finish. In your scenario the easiest way to validate the task
duration is to look at the task Start and Finish fields (include the
time). In my test of your scenario, the task start is 5/22/08 at 8:00 am
and the task finish is 6/3/08 9:00 am. The working time difference is 72
hours and given a setting of 8 hrs/day for duration calculations, that
is 9 days exactly.

Probably the best way to see the above is via the Task Usage view, with
the timescale set to show days and hours.

Hope this helps.
John
Project MVP- Hide quoted text -

- Show quoted text -

Thanks John. Your reply certainly helps. However, how can we validate
the formula Duration = Work/Units. I am unable to figure out how in
this case, the Duration of 9 days (or 72 hours) equals the Work(65
hours) dividied by Units (50%).

 

[Markarina]

Thanks John. Your reply certainly helps. However, how can we validate
the formula Duration = Work/Units. I am unable to figure out how in
this case, the Duration of 9 days (or 72 hours) equals the Work(65
hours) dividied by Units (50%).

I'm not pretending to be an expert here, but the way I understand it
is that when you set units to 50% you restrict the number of hours a
resource can work in one day not how the effort is split, so in this
case when you have two resources as has been previously said by
default project splits the effort equally so each resource has 32.5
hours of work. Given that you have set units to 50% this means your
8hr/day resource can work only 4 hours/day and as such the duration
will be calculated as 32.5/4 = 8.125 days which in this case means
project will report the duration as 9days.

 

[Rod Gill]

The way I see it is that you have effort driven on, so assigning two
resources each at 50% means they both share the work, each has 32.5h. As one
resource has 9h/d they will finish their work quicker than the other
resource. The task's duration is therefore driven by the resource that will
take the longest to finish.

As Jim says, look at the task Usage View to see this.

Duration=Work/Units=32.5/0.5=65h

At 8h per day the duration is therefore 8.125d

--

Rod Gill
Microsoft MVP for Project

Author of the only book on Project VBA, see:
http://www.projectvbabook.com

 

[Steve House]

Don't forget each resource is treated independently of the other. The
formula D=W/U does not apply to the aggregate but to each resource
individually. The overall task duration, on the other hand, is from when
the earliest resource begins until the latest resource end. For example, we
can have a task that has a 4 week duration with 2 resources each doing 80
hours at 100%. Normally that would result in a task duration of 2 weeks BUT
if Resource 1 works by himself weeks 1&2 while Resource 2 works alone on
weeks 3&4, the duration becomes 4 weeks, extending from when R1 starts until
when R2 finishes.
--
Steve House [Project MVP]
MS Project Trainer & Consultant
Visit http://project.mvps.org/faqs.htm for the FAQs


....
Thanks John. Your reply certainly helps. However, how can we validate
the formula Duration = Work/Units. I am unable to figure out how in
this case, the Duration of 9 days (or 72 hours) equals the Work(65
hours) dividied by Units (50%).

 

[John]

Thanks John. Your reply certainly helps. However, how can we validate
the formula Duration = Work/Units. I am unable to figure out how in
this case, the Duration of 9 days (or 72 hours) equals the Work(65
hours) dividied by Units (50%).

Ritesh,
Ok if you really want to know how we get 72 hours of duration, here is
how it is derived. I'm going to use the empirical approach rather than
try and derive it using Project's work formula. Obviously Project uses
the work formula to calculate the value but I honestly haven't taken the
time to work it out from an analytical approach.

The best way to see how the duration plays out is to display the Task
Usage view with the minor timescale set for hours. Keep in mind that
what is displayed is work hours, not duration hours, so the data in the
view requires some careful study.

The thing that throws a monkey into the intuitive approach is that one
resource works 9 hour days. The other thing that is relevant is that as
I stated before, the total task work is divided equally between the
resources and then spread linearly over a span of time.

As you look at the timescale data, each "day" the 9 hour resource works
an extra half hour over the standard 8 hour day. Add up the number of
days both resources work the task. That sums up to 8 days (64 hours).
During that time, the 9 hour resource also works a total of 7 one half
hour periods over the standard work day. Even though the work content is
only 0.5 hours, because the resource is assigned at 50%, that extra time
amounts to 7 more full hours of duration time. Meanwhile the 8 hour
resource works one half hour on the 9th day, but again, because he is
assigned at 50%, that is equivalent to 1 full hour of duration. Now,
adding up all the duration time we have 64 + 7 +1 = 72 hours. For a
Tools/Options/Calendar setting of 8 hours per day, that is 9 days
exactly.

From a realistic point of view, the actual duration is 8.125, (as stated
by Markarina in his/her response), but given Project's calculation
algorithm for duration, the duration is calculated at 9 total days.

Things aren't always obvious, particularly when multiple resources
and/or resources with non-standard work calendars are assigned to tasks.

Hope this helps.
John
Project MVP