Step 3: Activity Duration Estimating is the determination of the time required to complete the activities that make up the program. This is one of the most difficult aspects of schedule development and should be performed by people who are most familiar with the activity. Two key inputs to the estimation process are the resources required and assigned for the activity and the capabilities of the resources assigned. Historical information (Lessons Learned and Best Practices) from other programs and from commercial databases can also be helpful in developing accurate estimates.
The following techniques are commonly used in estimating activity durations: 
- Expert judgment guided by historical information,
- Analogous estimating based on the experience of similar programs,
- Parametric estimating based on formulas describing relationships among program parameters and time, and
- Use of simulation to develop distributions of the probable duration of each activity.
The output of this step is an estimate of the likely amount of time to complete each activity. These estimates should also include a range of possible values, e.g., 3 weeks ± 1 week, and a clear statement of the assumptions made in the estimation process. 
AcqLinks and References:
-  Defense System Management College “Scheduling Guide for Program Managers” – Oct 2001
- DoD “Integrated Master Plan and Integrated Master Schedule Preparation and User Guide” – 21 Oct 2005
- CDC “Project Scheduling Best Practices/Guide” – 30 Jun 2007
- GAO 12-120G “Schedule Assessment Guide: Best Practices for Project Schedule” – May 2012