A first-draft schedule almost always represents the way projects would go in an ideal world. But you live in the real world, and just as the jeans you buy never look like they do on the catalog model, your real-world project will have a few bumps and wrinkles. Understanding resource units (Defining When Work Resources Work) and calendars is only half the battle. Now you have to figure out how to combine them to handle real-world resourcing situations. This section explains how to make Project's resource features model reality.
Part-time workers don't work full days, so they take longer to finish tasks. Someone working half-days takes twice as long, because you only get 4 person-hours a day, instead of 8. To make sure you don't over-assign part-timers, you must adjust the resources' maximum units and, in some cases, their calendars. Then, when you assign them to tasks, Project automatically doles out time based on their availability.
Here's how you handle part-time workers in Project:
Set the resource's maximum units. When someone works the same amount of time each workday, change the Max. Units field (Specifying how much time resources are available) in the Resource Sheet. Click the resource's Max. Units cell, and then type the percentage that the person works. For example, the maximum available units for someone who works half-time is 50 percent.
Change the resource's work schedule. Project assumes that resources are available ...