Often I am asked by Project users about assignment units and how can you explain the behaviour in MS Project, most often the questions are around why the % assigned units of a particular resource assigned to a task (an assignment) can change unexpectedly.

The situation can be explained through the following scenario using MS Project 2007:

  1. A Project Manager creates a task of 10 days, assigns a resource and sets the Units to 20% (15 hours of work with 1.5 hours planned per 7.5 hour day) using the task settings of Fixed Duration and Not Effort Driven.

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  1. Now the Resource submits a timesheet that contains Monday = 1.5h, Tuesday = 0h, Wednesday = 1.5h.
  2. After the status update is accepted into the plan the resource Assignment Units has changed to 23% and a gap is shown in the Gantt chart bar.

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This behaviour is expected. The cause is due to the missing day (Tuesday) of work which results in an additional 1.5h of work being spread across the remaining days of the task resulting in each remaining day now having 1.72 hours of work, or put another way a ‘peak’ assignment units of 23%.

This can lead to some confusion when looking at project schedules as if like many people your organisation assigns resources to work on such % allocations, typically through an approval process which dictates the "maximum" % allocation for any given resource.

The solution for the PM of course is to reduce the remaining work to return the peak units back to 20% ie in the above example subtract 1.5h work from the total. To see this do the following to correct the units in the above example:

  1. The Project Manager now edits the remaining work for the given task and reduces it by 1.5h, now the Units returns to 20%.

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What’s new in Project 2010?

This behaviour has changed significantly in Microsoft Project 2010! In fact the behaviour now is closer to what you might expect; Microsoft has introduced a new field in addition to Assignment Units you now have Peak Units which shows exactly what you might guess from the name in the example above, in short you can see both the original “assigned units” of 20% and the actual “peak units” of 23% together.

See the following quote from Microsoft:

"In Project 2007, and previous versions, when this value differs from 100% we show it next to the resource name in the Gantt chart. For Project 2010 we’ve made some changes to the way that the Assignment Units field is calculated. Primarily, these changes were made in response to customer feedback about the way calculations were impacted when resources entered overtime work. For this release we’ve clarified the definition of the Peak field and the Assignment Units field which previously had some functional overlap but now fill more defined, separate, roles. As a result of these changes the Assignment Units field is no longer automatically modified to be greater or less than default value of 100%; as a consequence the field does not show up in the Gantt chart as often as it used to."

Source: Assignment Units in Project 2010

For more information on task types, assignments and such here is another good read on the topic;

Task Types: Don’t Get Frustrated!

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