There is a fundamental formula that ties together the duration of an activity against the amount of work effort required, and the resources needed to complete that work within the estimate activity resources process:

Activity duration = work effort/human units. For typical activities the duration will be measured in days, the work effort in hours and the amount of people required as a decimal fraction.

This particular process focuses on each activity within the schedule to calculate the resource requirements. Another factor that will need to be considered along with the hours work effort required, is the availability of the resources.

All of the above will combine end result in the project schedule which is developed in a later process within the time management group.