Reason number one is that in all but the most extraordinary situations it is modeling a situation that is just not possible.
But first some background on what Max Units really is. Max Units defines the percentage of a resource’s full calendar working “period” that they can be assigned to work on tasks before Project sees them as being over-allocated.
A resource’s calendar says they come in at 8am and work until 5pm and take a 1 hour lunch. They do this Monday – Friday. That is an 8 hour work day\40 hour work week. So if the Max Units is 100% then if the resource is assigned to work 9 hours in one day they will be seen as over-allocated. Same for if they are assigned to work on 2, 1 hour tasks during the same hour. This goes down to the minute level too so if two 1 hour tasks overlap by 1 minute then for that 1 minute they are over-allocated.
This helps the PM create models of assignments and get an idea of how many hours each team member is being assigned to tasks and how that falls across time, other assignments, etc.
So now you might be seeing what is wrong with 100% Max Units. It says that if I work an 8 hour day, I am available to work 8 hours on tasks. On it’s face this sounds logical but dive a little deeper and it becomes obvious that this is just not possible. Nobody ever arrived at work at 8am, took a 1 hour lunch, and then left promptly at 5pm AND got 8 hours of work done on tasks. EVER.
OK wait, I take it back. It is possible that someone did this on your project: IF your project schedule has tasks for things like: going to the bathroom, answering non-project related emails, going to a company meeting, being tapped on the shoulder by your cube-neighbor and being asked for “just a quick 5 mins. of help” (that turned into 30 mins), the list goes on and on. So if your project contains a task for every possible distraction from YOUR project and you expect your resources to track all of that then never mind. You can set your Max Units to 100%. (just count on a lot of churn on your team.)
But for most of us it is not possible to work 8 full hours on PROJECT WORK in an 8 hour day. Doing so means that you were present for more than 8 hours so that all the other things had time in your day along side your real work. The best way to help our models (because that what project schedules really are: models of what we want our project work to look like) be more accurate is to lower Max Units to something more like 85%. That would be the highest I would ever go on any project I was managing. I have seen it set as low as 75% at some sites but generally I see 80-85%.
What this means is that if you have a 1 day duration task and you assign a resource that has an 85% Max Units value, Project will calculate the Work for that task to be 6.8 hours. This means that you are modeling that on average this resource spends 1.2 hours of their 8 hour day doing something OTHER THAN working on your project. A Max Units value of 75% means that 2 hours is spent doing other things. Of course some will get more than 6.8 done in a day and some will get less done. It depends on the nature of their job, their relationship with other projects, other teams, etc. So the value you set will never be perfectly accurate but it WILL certainly be MORE accurate than 100% which is nearly always wrong. The point here is to make your model as accurate as you can.
I’m hoping to start a small series of Deployment Practices posts here covering things I have found to be useful ideas, practices or methods for deploying Project Server. Please email me if you have suggestions or questions.