Simple examples of the difference between fixed-work, fixed-unit, and fixed-duration tasks in MS Project

Question

MS Project tasks are either fixed work, fixed unit, or fixed duration, where

Duration x Units = Work

The documentation describes which variable is recomputed when one of the others is changed, but I can't think of simple examples to help me decide which I should use.

Edit: I'm not interested in the task types you typically use or a heuristic for selecting one, because—like the official help documentation—this is too abstract. I'm not looking for a rule of thumb but an example-based framework for understanding how the task types are intended to be used. For example:

• painting a fence is a fixed-_________ task because…
• writing a test report is a fixed-_________ task because…
• etc.

Answer 1

Fixed-duration example: Program Management Level-of-Effort

Not a work item directly associated with the project product but one that supports it; its duration is based on the duration of the activity it is supporting.

Because duration is fixed:

• more units (i.e. more project managers) means more hours spent managing.
• less work means less units are required.

Increasing duration means the management supports the project longer and thus does more work.

---

For fixed-unit and fixed-work tasks revising either units or work changes the duration. They differ in what's recalculated when duration is changed.

Fixed-unit example: writing a report where the depth of the analysis is flexible.

Paul's analysis skills are so specialized only he can write Report X. If the duration changes then the scope of the report (i.e. the work) changes with it so he can finish on time.

Fixed-work example: painting a fence.

How long we have to paint the fence determines how many people need to work on it. Less time means a larger crew of painters.

Answer 2

Loading 1000 bags of cement into a truck is a fixed-work task.

Gestation is a fixed-duration and fixed unit task. It takes 100% of one resource (fixed-unit) and 9 months (fixed-time)

Burning 10.000 copies of a finished software is a fixed-work task.

Testing a process workflow may be a fixed-time task, if you have to actually wait for real events to happen (i.e.: accepting a purchase order, shipping and receiving the delivery confirmation)

Fixed-unit examples are harder to find, but usually are the tasks where you use a fixed percentage of a resource, as in the pregnancy example.

Answer 3

In my opinion, it is not about finding tasks that fit which variable you fix, it is about fixing a variable in order for the auto scheduling, leveling, etc., in project to work. It requires a variable to be fixed in order for it to behave a certain way. So you would choose which variable to fix in any given package so that when you allow project to auto schedule it will behave in a predictable.

For me, it is more about the sort of mental model I have as I am sequencing my resources that are assigned to the packages. Because of that, I schedule almost exclusively using fixed duration. In other words, I do not really care what project does to work or utilization. The very stochastic effects that occur in real project work make tracking work and utilization, e.g., an "over utilized" resource, futile. What I worry about are deadlines. I need to know how progress, or lack thereof, is affecting my target deadline based on my target value I used for duration. This makes the most sense TO ME so I want project to behave in particular way with duration in mind.

So, IMHO, do not worry about tasks that fit the fixed variable; instead, think about what is logical to you and then determine which variable you need to fix in order to have project behave the way you want. It is a very clugy tool so you need to think about that carefully.

Answer 4

I generally approach task types by asking which of the three variables - work, duration, or resource units - is my most limiting factor.

If I am fairly certain that I am unable to negotiate for more resource time (increased units) I will work with Fixed Units task types. With a fixed unit task type if the work increases, the duration of the task must increase.

If I am more confident of the work estimates, I may choose to set a fixed work task type. Then if the resource asks for addition time (duration) to complete the task, the work remains steady and Project recalculates peak units.

If, like David, the issue is time and I can negotiate resource available or scope (work) then fixed duration makes the most sense.

Rarely do I have an entire Project with only one task type. The caution with both Fixed Work and Fixed Duration is you can overallocate a resource on a single task and the Resource Leveling command will not resolve the issue.

Answer 5

I'm new to Project but I'll venture a task as an example of Fixed Work for you:

It's an IT task to develop a suite of programs.

The work estimate is roughly reliable (although it may change if new information comes to light during the task). So I will aim to set and meet the deadline by managing resource units. However, the work is complex and focussed tightly on each single program so there is a limit to how many units can work on a task together effectively. Therefore I cannot make the duration fixed. If we overrun, we have to either deliver late or drop some of the project from the first delivery.

The initial units to be assigned is known up front (although this may also change, for example if other higher priority projects draw resources away, or if work increases and we need to meet the project deadline.)

The duration is something we want Project to track for us, so that we can see if we are meeting the project deadline as things stand.

I'll set the task up as Fixed Work and assign available Units to it, then see the expected Duration.

If the duration is unacceptable, I'll add units as far as I can. The deadline is then set.

If units are withdrawn during the project for more urgent projects, I'll see the duration increase, unless I can add more units.

If a unit meets with a problem or discovers hidden work, we'll increase the Fixed Work accordingly, and see the duration change again. Hope this helps. I am so new to this I amy be barking up the wrong tree entirely. :)

Answer 6

For my consulting IT work, "Fixed Work" is usually the type I go with. Schedules and resources are more flexible for us, but the financial aspect related to firm proposals, and staff hours available to meet that proposal, are critical.