early start and finish times
To perform the forward pass on a Precedence Diagram two ingredients are needed. The first is the starting time of the project. Many projects are scheduled according to work days and weather impacts are not considered, therefore, unless otherwise instructed, all projects may be assumed to start on day one (1).
The second component needed to perform the forward pass is the complete set of activity durations. Activity durations and sequence should be determined prior to drawing the precedence diagram and, therefore, should be available as you perform the forward pass.
The diagram below shows a simple three activity schedule with Early Time boxes ready to start the forward pass.
To conduct the forward pass in the Precedence Diagram, start with the first activity in the schedule and place the project start date in the Early Start Time. The Early Finish Time of each activity is the Early Start Time plus the activity duration. The Early Finish Time of a given activity becomes the Early Start Time of its following activities. The forward pass is completed when every Early Start Time box on the network has a value.
The figure below begins with the Early Start Time of Activity 1 being set to the project start time of one (1). The Early Finish Time of Activity 1 is equal to the Early Start Time of Activity 1 plus the duration of Activity 1. Following this pattern we can see that the project will require 19 working days to complete. Regardless of the technical differences between the Arrow and Precedence Diagrams the results of schedule calculations will be the same regardless of which method is chosen.
Most schedules are, of course, not as simple as that shown in the diagram above. There are, however, clear rules about how to calculate Early Event Times in networks where there are multiple activities starting and finishing at nodes within the schedule.