Allowing for Task Delays and Overlaps | Mastering Microsoft Project 2002

Even though two tasks may be linked, the planned start or finish date of a task may depend on a number of other factors. In a finish-to-start relationship, finishing a predecessor task doesn’t necessarily mean that you can start the successor task immediately. If you’ve ever flown on a commercial airline, you’ve probably experienced this firsthand. Airlines routinely include lag time—a delay between landing and taking off again—in their schedules. This allows time for deboarding, refueling, exchanging baggage, restocking the kitchen, cleaning the cabin, and reboarding.

Note

In the real world, there’s often a difference between a flight’s scheduled arrival time and the time it is expected to actually arrive. Even if a plane arrives later than what the airline expects, it may be well within the publicized arrival time. “Stretching” the duration to improve the airline’s on-time arrival percentage contributes to overall customer satisfaction, and improves the airline’s efficiency rating.

Understanding Lags and Lead Time

Any delay between any two dependent tasks is called lag time.

In Figure 8.17, Tasks 26 and 27 provide an example of lag time. In this example, the marketing team is planning to request business directories for the local community and then use those materials to identify major competitors. Although the task of requesting directories should take only two days, it may take more than that for all of the directories to arrive. By building in a five-day lag time, you can more accurately reflect the expected lag time between requesting the directories and beginning the process of identifying competitors.

Figure 8.17: Lag time (tasks 26 and 27) and lead time (tasks 29 and 30)

On the other hand, if a successor task can start before its predecessor is finished, this can be demonstrated through the use of lead time. Lead time, then, is the overlap between dependent tasks. Let’s say, for example, that one task is to identify target clients of the new regional sales office. Based on the results of this process, the marketing team plans to create marketing materials. As long as you allow some lead time to establish a general sense of the target clients, the marketing team can begin drafting marketing materials. Tasks 29 and 30 in Figure 8.17 show this relationship.

Entering Lags and Lead Time

Lag time and lead time are both entered in the Lag or Predecessors fields. Lag time is entered as a positive number, and lead time as a negative number; entering 5 indicates a lag time of five days, while entering ñ5 indicates a lead time of five days.

Lag time and lead time can also be entered as a percentage of the predecessor’s duration. A 40% lead time, for example, means that the successor task would start when the predecessor is 60% completed. Use a percentage when you want the lag or lead time to increase or decrease proportionately if the duration changes.

You can enter lag time and lead time by using the Task Information dialog box, Task Form view, the Task Entry table, or the Task Dependency dialog box.

Using the Task Information Form

To use the Task Information form, follow these steps:

Double-click a task to open the Task Information dialog box.
Click the Predecessors tab.
Select the Lag field.
Enter a number or percentage in the cell, or use the spin arrows to increase or decrease the lag or lead time.
Click OK to close the Task Information form.

Using the Task Form View

To use Task Form view, follow these steps:

Click Window Ø Split in Gantt Chart view to open the Task Form.
If Predecessors are not visible, right-click on the Task Form and then choose Resources & Predecessors, Resources & Successors, or Predecessors & Successors.
Select the Lag field.
Enter a number or percentage in the cell, or use the spin arrows to increase or decrease the lag or lead time.
Click OK on the Task Form to record the changes.

Using the Task Entry Table

To use the Task Entry table, follow these steps:

Drag the vertical split bar to the right to display the Predecessors field.
Select the field, and edit the predecessor to include the lag or lead time information. You must also enter the abbreviation for the relationship type, if it is not already present. Figure 8.18 shows one example of five days of lag time (26FS+5 days) and one example of 40% lead time (29FS-40%).

Figure 8.18: Enter lag and lead time in the Task Entry table.

Using the Task Dependency Dialog Box

To use the Task Dependency dialog box, follow these steps:

Double-click the connecting line linking two tasks in the Gantt chart to open the Task Dependency dialog box.
Select the Lag field.
Enter a number or percentage in the cell, or use the spin-box arrows to increase or decrease the lag or lead time.
Click OK to close the dialog box and save the changes.