Using Leads
Leads, in particular, are often unnecessary. As negative lags, leads imply the unusual measurement of negative time and exact foresight about future events. In the example in figure 18, management must assume some prescience to start installing siding and brick finishes 1 day before doors and windows are installed. Any delay in installing doors or windows within the last day may cause immediate problems for the plan. A 2-day look ahead might seem trivial, but leads of weeks or even months are unfortunately common in some schedules, questioning people’s credibility when forced to see that far into the future on a risky project. In effect, a lead indicates that a future event will dictate the timing of an event in the past, which is neither logical nor possible.
The concept of a lead is better represented by a positive lag on an S-S relationship or, even more straightforward, on F-S relationships with no lags using activities of shorter duration. For example, instead of a lead, the installing doors and windows activity should be broken into smaller activities to identify the proper F-S activities. For example, the doors and windows activity might be broken into two sequential subactivities: “install exterior east and west doors and windows” and “install exterior north and south doors and windows.” “Install exterior siding and brick finishes” could then be linked with a finish-to-start relationship to “install exterior east and west doors and windows,” as shown in figure 21.
Figure 21: Eliminating Leads with Finish-to-Start Links
The network now clearly shows that once the east and west exterior doors and windows have been installed, installing siding and brick finishes can begin. It also clearly identifies that siding and brick finishes installation can go on while doors and windows are being installed in the last two sections of the house. Finally, using leads improperly can cause logic failures when a lead is longer than the successor activity. An example is given in figure 22.
Figure 22: Logic Failure Associated with a Lead
The F-S link in figure 22 between “finish drywall” and “apply drywall texture” dictates that finishing the drywall must finish before applying the drywall texture can begin. Suppose the relationship also has a 4-day lead that states that applying the drywall texture should begin 4 days before drywall is finished. However, applying texture takes only 3 days, which means this activity will actually finish before the drywall is finished. This causes a conflict in logic between the lead and the finish-to-start relationship. Moreover, because the lead causes texture application to finish a day earlier than the finishing of drywall, an artificial day of float may be introduced into the sequence of activities.
Note that the number of activities affected by a lag or lead is different from the total number of lags or leads in the schedule. The total is useful for determining the extent to which updating the schedule will be affected by the use of lags and leads. As stated above, a major disadvantage of lags and leads is that they are static. If they are prevalent in the schedule, the update process requires significant manual effort and becomes time consuming and prone to error.
The total number of predecessor relationships with a lag or lead provides a scheduler with a sense of how cumbersome updating will be. If the schedule is not well maintained, its utility as a dynamic management tool will be reduced and activity dates may be artificially delayed or accelerated by a static lag or lead. For this reason, it is also worthwhile to understand the number of activities that will be affected by such a situation. For example, figure 23 gives an example of three concrete-pouring activities. Each activity has a 2-day lag from its predecessor to represent the time needed for curing. But because “form and pour driveway” takes a day longer than “form and pour sidewalks” and “form and pour patio,” only its lag directly affects the start date of “finish grade property.”
Figure 23: Enumerating Lags
