The Critical Path Method (CPM) - Construction Project Scheduling and Control - page 72

Lead is a negative lag. Thus, a lag means “after” and a lead means “before.” Although
lags are not real activities, they consume time and must be incorporated into the CPM
calculations. They are shown as numbers above the lines of arrows: these numbers may
be boxed (as in Figure 4.12) or not boxed (as in Figure 4.10).
You can think of a lag, if this simplifies calculations, as a real activity, in which
duration equals the lag. This works well for positive lags but not for leads (negative
lags).

Example 4.4

Redo example 4.2 with some lags added as shown next.

Activity IPA Duration Lag

A - 2

B A 6

C A 10

D A 4 4

E B 7

F B 5 3

C

G C, D 3

H E, F 5

I G 2

H 1

Solution

Performing forward and backward passes yields the solution shown in
Figure 4.10. The 4-day lag between activities A and D means that activity D
cannot start until at least 4 days after the completion of activity A. The same
principle applies to the lags between activities B and F and between activities
H and I.
In this example, the critical path does not change, but it may change
in other cases. The 1-day lag between activities H and I adds 1 day to the
project. In fact, if we look at the definition of critical path (the longest path in
the project network from start to finish), we need to include the lags as part
of the path. In this example, the critical path is A, C, F, H, I. Its length (i.e., the
duration of the entire project) is 2 + 10 + 5 + 5 +( 1 )+ 2 = 25 days (the lag is
in parentheses).
Other lags in this example contribute to changes in the forward-pass and
backward-pass calculations, causing total float and free float to change in
some cases. Notice that in the CPM calculations, the lags must be added in
the forward pass and subtracted in the backward pass.