The first step in determining the basic schedule is to do a forward pass. The forward pass gives you the total duration of your project. You start with the very first activity and plug in a zero for its early start date. Then add the duration to the early start date to get the early finish date. The early finish date for an activity becomes the early start date for the next activity. Notice that activity 1050 in figure 2-13 had two preceding activities (1020 and 1040) and you chose the larger of the early finish dates (11 vice 10). Remember to add any lag between the activities (between activity 1070 and the next activity there are 3 days lag time). Lag times are mandatory wait times between activities. A common example is concrete cure times. Cure times require you to wait several days to several weeks after placing concrete before you strip the forms. Follow the following two equations through the network in figure 2-13:
Early Start + Duration = Early Finish
Early Finish + Lag (if any) = Early Start (next activity) *
Look at the network in figure 2-13, the early start and finish dates for an activity depend on the number and duration of the activities that have to be done before it.
The next step in determining the basic schedule is a backward pass. The backward pass determines your critical path. You start by taking the early finish date for the last activity and making it the late finish for the last activity. For each activity, subtracting the duration from the late finish date will give you the late start date. The late start date will become the late finish date for the preceding activity. Notice that activity 1040 in figure 2-13 has two follow-on activities and you took the smaller of the late starts (11 vice 12). Follow the equations shown through the network in figure 2-13. For any activity where the early start is the same as the late start and the early finish is the same as the late finish, that activity is critical!
Late Finish - Duration = Late Start
Late Start - Lag (if any) = Late Finish (preceding activity)*
Total float is the number of days an activity can be delayed without delaying the project completion date. Looking at activity 1020 in figure 2-13 you see that it could finish as early as day 10 or as late as day 12. The 2 days of leeway between day 10 and day 12 in activity 1020 are called total float. To calculate total float you subtract the early finish date from the late
Figure 2-13. - Typical network.Continue Reading