Question on how to assign a person to multiple projects/tasks?

I have several tasks (or projects) assigned to one person (during the same period). But it will create an error in resource sheet: "This resource should be leveled based on a day to day setting". But when I level it using day to day setting, the project will be automatically delayed/extended... This is not what I want either. If I have start date, end date, and maybe persentage of the time that person should be spending on each project during a certain period.  How could I assign a person to multiple tasks (or projects) during the same period without extending the project, etc.? I thought it's a quite common scenario, but I've been searching for a while and still didnt get anything valuable. Thanks.
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You can assign a person with a percentage of their time. Project will assume that every person works 8 hours a day, 40 hours a week unless you provide a different schedule. It also assumes when you assign someone to a task that they work on that task 100%. So by default when you assign someone to two tasks on the same day, Project would think they are working 16 hours which is above the normal schedule.

When you are doing your assignments, there is a column in the Assign Resources dialog box for MAX UNITS. This is where you define the percentage of time that they are to work on the task. For example if you wanted one resource to complete 2 tasks in one day, you could assign the person to each task with a 50% Max Units. This results in a 1 day duration but only 4 hours of work. You can adjust percentages to meet the actual assignment details of the multiple tasks.

Hope this helps.

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I'd like to suggest that you never enter "start date" or "end date" for any task.  You simply need to enter "Project Start Date" (Project -> Project Information, in the menu) and then your task list with their "Duration".  Then you link all tasks together, indicating their sequence.  You do this by selecting them in pairs and pressing the Link Tasks command in the toolbar (or Ctrl-F2 as a shortcut).

This way none of your dates will be fixed, and you can benefit from the power of Project to recalculate all the schedule every time you need to change the Project Start Date, or that any task is modified.

As mentioned by dbase118 you can specify the percentage of "units" that a resource will spend on every assignment.  But it will be natural that for any assignment you do at 50% (for example), the duration will be extended to be twice as long.
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