Let’s consider the following situation: Your project has been running behind schedule for the past few weeks. You have done everything in your power to rectify the situation and bring the project back on track, but alas, your efforts have fallen short. Your customer is quite peeved at you, and you just do not know how to handle the situation.
Finally, you decide you can bring the project back on track by adding few more resources. You convince your manager to assign these extra resources to the project, and since the project and the customer is very important for your organization, your manager agrees . However, he asks you to show him a revised schedule before proceeding. You’re hoping you can revise the schedule in short time.
You quickly open the current schedule in MS Project (MSP) and start adding resources to the tasks expecting that MSP will automatically reduce the assigned durations. But, MS Project starts to change the schedule in its own way. You are quite bewildered because MSP doesn’t reduce the duration. In fact, it changes nothing. You probably keep working on the MSP schedule for some more time without making any headway.
After a few hours, you may start to become frustrated. You wanted to show the revised schedule to your manager as soon as possible, but the way MS Project is acting, you are not sure how long it will take. What now?!
Scenarios like these likely happen because of the way MS Project defines its tasks. In this article, I’d like to go into the three types of MSP tasks and their practical utility.
Task Types in MS Project
MSP allows you to define a task in one of the following three ways.
- Fixed Units
- Fixed Duration
- Fixed Work
Additionally, it allows you to make tasks “Effort Driven,” as needed. In MSP, work and effort are synonymous, and by default, MSP defines a task as a Fixed Unit. Let’s look at the definition of these terms:
- Units signify the number of resources assigned to a task. They are usually represented as a number or percentage.
- Duration is the length of time required to complete an activity. It is usually represented in hours or days.
- Work is the effort spent on an activity or task. It is usually represented in person hours or person days.
The relationship between the above terms can be defined with the following formula: Work = Duration * Units.
For example, if two people are working or a job for three days, then the work becomes six (person) days. You can look at my previous article to understand how to setup or change task types in MSP.
Fixed Units
This type of task is used when you have limited resources for a task or when adding extra resources to a task will not be beneficial. For example, only one developer can be assigned to creating a particular piece of code. You cannot decrease the development time by adding extra developers.
This piece of code will take four days to complete if the developer works full time on it, but if this developer is assigned to work 50% on the task, it will take him eight days to complete the task.
Fixed Work
Fixed work is a type of task used when you have done the effort estimation of a task and resources can be interchangeably used for it. For example, by assigning extra testers you can reduce the duration of system testing a software module.
Let’s assume that you have estimated an effort of 20 person days to complete system testing. This work can be completed in one of the following ways:
- Assigning 5 resources for 4 days
- Assigning 4 resources for 5 days
- Assigning 2 resources for 10 days
- Etc.
Fixed Duration
A fixed duration task is used when you have been given a time constraint by the client or senior management. It doesn’t matter how many resources you use, but you have to finish the task by a certain predefined deadline. For example, releasing a software module before the New Year.
Recalculation Examples for Different Task Types
Let’s revisit the relationship formula, which is Work = Duration * Units.
MSP uses this formula to automatically rejig the tasks. The following table shows how MSP recalculates the schedule, when you change something.
Let’s consider a task that is planned to be completed in three days by two people.
- Units = 2
- Duration = 3 days
By applying the relationship formula, we get: Work = 6 person days
The following table provides a quick overview of MSP calculations for a Fixed Work task:
And this table provides a quick overview of MSP calculations for a Fixed Units task:
You can also make changes in Fixed Duration tasks. Here’s an overview of MSP calculations for that type:
Conclusion
There are three different task types in MS Project. Each of the tree different task types in MS Project is important in its own way. You should choose your task type carefully while creating MSP schedule lest it give you problems later.
When you need to make changes, look at the task type before changing the parameters of it. Otherwise, you may accidentally change something which may be difficult to undo.
Which type of task do you generally use in your MSP schedule and why? I would love to hear your comments below, and if you’d like to learn more, check my webinar, MS Project Task Types: A Practical Perspective.
Daryl Deffler
MS Project calculates task duration using a basic formula: Duration = Work / Availability
In this formula, “Availability” means the amount of work units the resource is available. For example, 50% availability on a 40hr week is 20 hrs/wk. If a task’s Work was estimated at 80 hours, in this example, the Duration would be calculated at 20 days (80hrs/4hrs/day) or 4 weeks.
This formula must always be in balance for each task and after task changes or during leveling, Project will, without telling you, change values to ensure the formula is balanced.
The “Fixed” designation on task type identifies which of these three formula components is locked, meaning Project will not change that component during calculations. This is illustrated by the first graphic in your article. Fixed Duration locks the Duration component, Work locks the work estimate component, and Units locks the Availability (or resource units) component.
Fixed units should be used when the AVAILABILITY of a project resource should not be changed by the task duration calculation. For example, Joe is only allocated to the project 50% of his time or 20 hours a week (based on a 40 hr wk). He’s assigned a 40 hour task and with Fixed Units, the task duration will be two weeks.
If we change the task type to Fixed Duration of 5 days (PM enters Duration = 5), MS Project would (without telling us) change Joe’s task assignment units to 100% to balance the formula. Meaning, Joe would need to be 100% available to the project for that week. And since Joe is only available to the project at 50%, the result is an over allocated resource and an unrealistic schedule.
In your article, you define Fixed Units: “This type of task is used when you have limited resources for a task or when adding extra resources to a task will not be beneficial.” Almost.
To tweak that definition, it should say “when you have limited resource AVAILABILITY”, period. The whole part about adding an extra resource or not is something the PM determines. If its shoveling dirt, an extra resource will help complete the task quicker. If its something like writing code that only one person can be doing, then it doesn’t make sense to add another resource. But that’s what the PM determines and has nothing to do with Fixed Units.
Ronald Smith
I enjoyed reading the article and especially Daryl’s comments, but at the same time I can’t but wonder about the big picture of what is going on. For example:
1. Can the scope be reduced?
2. Are there any tasks that covers work not included in the scope statement?
3. Are you looking at the critical path tasks first to see if you can shorten the schedule?
3. Any duplicate tasks or tasks out of sequence?
4. Any underestimating of delivery dates?
5. Has fast tracking been considered (doing activities in parallel that you would normally do in sequence) to shorten the schedule?
6. Any change requests taking place or planned for?
7. Are the Stakeholders and Sponsor meeting their commitments? Have you met their communications needs?
8. Have your task buffers (e.g., at the end of a difficult phase like using new technology) been used up?
9. Are you being careful about the Law of Diminishing Returns? At some point, adding more resources may increase the total time (and costs) because of extra
risks, knowledge transfer, and additional coordination.
Tammo
Great article. After more than 50 years in the engineering and construction profession, I have just about always used “fixed duration” because it is easier to deal with. In scheduling, the durations are the key item of interest. The resources are merely there to make it happen. So, if we were concerned about the availability of a resource, we covered that by analyzing the resource assignments/over allocations and if we needed to apply more resources, we made sure we got them, and if they were simply not available, we’d adjust the duration manually.
This way, the schedule (that is, the timing/dates) were not affected unexpectedly, as your article suggested. I teach a course on MS Project and I use fixed duration throughout, except to cover the task types.
Your coverage of this subject is excellent and, as you say, there are times when the fixed units or work types might come in handy. Thank you for taking a lot of the confusion out of the topic. I particularly like the last 3 tables in y our article.