Relationship between Duration and Work fields in MPP

Question

In my MPP setup, I have:

• a resource, R1, available for my project for 50%, and;
• a resource, R2, available for 100%.

So I marked them in the "resource sheet" as "Max. Units" of 50% and 100% for R1 and R2 respectively.

When I create a task in "Gantt chat view" and assign to R1 a task with a duration of 4 hours, it turns the "Work" field as 2 Hrs. When the same task duration is assigned to resource R2, it does not impact the "Work" field.

I am unable to understand the reason why.

I am now thinking that all of my project planning may actually be showing me wrong numbers in the total hours.

My understanding is: If a task is of 1 day duration; assigned to a resource with only 50% availability, it should actually change the "Work" field to 2 days.

I have attached an edited screen shot to understand the problem better.

Thanks.

Answer 1

Pardon me for bumping in. You are dealing with two different ideas. Duration and work. Duration is the value you enter in the duration field when you create the task. It is the answer to the question "how long is this going to take?". When you assign resources Project, unless told differently by you, calculates the work using the following formula:

Duration * assignment units = Work

So:

4h * 50% = 2 hours of work.

You can always enter the work value when you assign the resource. (split your screen to show the task form in the lower pane). If you tell project how much work, Project will recalculate the duration based upon the following formula:

Work/assignment units = duration

I hope this helps.

Answer 2

If R1, who can only dedicate 50% of their time to any task, does a piece of work that requires 2 hours of effort, then it will actually take them 4 hours to do it- It will take them twice as long because they can only work on it for 50% of the time. Therefore if the duration of a task is pre-set at 4 hours then the actual work (effort) to be completed by R1 can only be be 2 hours.

If R2 can dedicate all their time (100%) to a task and that task has a pre-set duration of 4 hours, then the work (effort) that R2 will do must also be 4 hours.

MS-Project works from the basis that tasks have an absolute amount of effort required to complete them which is wholly independent of the amount and type of resources allocated to the task. The more resources (people) allocated to do a task the smaller the expected duration as the same amount of work will be split up and worked on in parallel by a greater number of people.

If you start by entering the Work and then assign a resource of 50% then the Duration will double. If you start by entering Duration then the work will halve.

Answer 3

The major factor here is the type of the task and effort driven or not. You can make a task fixed duration, fixed work, or fixed units. I will ignore fixed units for this explanation. If a task is fixed duration and effort driven, then no matter how many people you assign to it, the duration won't change. For a fixed duration task however, if it is not effort driven, adding resources increases the amount of work required for the task without changing the duration. If you add resources for a fixed duration task, the utilization rate will decrease for the resources. A fixed work task is the opposite. Adding resources to the task will decrease the duration by spreading the work over the available resources. An example:

Task A requires 40 hours of work to complete and is effort driven with fixed work. If I assign Brian to do task A, it will take him 5 days at 100% utilization. If I want it done sooner, I can add Clark to the task. With Brian and Clark working together, it will only take 2.5 days (20 hours). Brian and Clark will both have 100% utilization.

Task B takes 5 days to complete and is a fixed duration and effort driven. If I assign David to the task with 100% utilization, it will be complete in 5 days and he will do 40 hours of work. If I then add Ethan to help on the task, it will still take 5 days. Now David only has to do 20 hours of work and Ethan does the other 20 hours. So both of their utilization rates are now 50%.

Task C takes 5 days to complete and is fixed duration but not effort driven. If I assign Fred to the task with 100% utilization, it will be complete in 5 days and he will do 40 hours of work. If I then add Greg to help on the task, it will still take 5 days but the work will increase to 80 hours. Both Fred and Greg will still have 100% utilization.

A fixed work task is always effort driven so there cannot be a case where there is fixed work but not effort driven.

You can change these settings by double clicking on a task and going to the "Advanced" tab. Here you can set the Task Type (fixed units, fixed duration, or fixed work) and effort driven or not. Hopefully this helps you understand how the scheduling engine works a little better. Good Luck!

** When a task is fixed duration and effort driven, and you add a resource (as in Task A) the resource utilization rate will not appear as a percentage in the resource column next to their name. It will show both of them as a regular resource with 100% utilization but they will only be working 20 hours each, so if you assign them to a second concurrent task with the same type, duration and work, there will not be an overallocation because they can work up to 40 hours. You can see how many hours of work a resource has by going to the resource sheet and viewing the "Regular Work" column.

Answer 4

I assume the task type is fixed unit. If R1 has standard calendar (8 hrs per work day) and assigned 50% to the project and you use day unit for duration, then he can do a 4 hrs task in 1 day; therefore ms project calculation is wrong and the reason is that task type is fixed unit. you can change task type to fixed duration or fixed work. both work right for your situation: enter work filed or duration filed as 4 hrs; other filed will be 4 hrs.

2nd solution is to keep duration unit as day and enter work filed as 4 hrs, duration will be 1 day as expected for R1.