Jonathan Yong

In the previous blog, I have explained about lag and lead time. In this blog, I will give you a scenario of how and when to use lag and lead, and how two schedule that use lag and lead although look the same, it is semantically different.

Consider the following two schedules.

Schedule 1

Figure 1: This schedule use a finish-start relationship with a 1 day lead time.


Schedule 2

Figure 2: This schedule use a start-start relationship with 4 days lag time.

In both schedule, Development task start on 6-Nov and Testing task start on 10-Nov and they both look the same. From the schedule dates point of view, they can be considered same. However, from the schedule semantics point of view, they mean different thing. In schedule 1, my schedule says that Testing should start one day before Development finish (the successor task is relative to the finish of the predecessor). In schedule 2, my schedule says that Testing should start four days after Development start (the successor task is relative to the start of the predecessor).

Now, let say I change my Development task to 8 days. Here is how the new schedules look like:

Schedule 1

Figure 3: Testing now starts on 15-Nov.

Schedule 2

Figure 4: Testing still remains to start on 10-Nov.

Now the dates in both schedule different. So, which is the correct way of scheduling? They are no right or wrong way, it all depends on what are you trying to express in your schedule (although in this case, Schedule 2 seem to be incorrect. How can you finish testing while development is still in progress?).

If your successor task is relative to the finish of the predecessor task, then use finish-start relationship with lead time. For example, you can only start testing towards the near completion of development. If you start too early, you might not have enough things to test.

If your successor task is relative to the start of the predecessor task, then use start-start relationship with lag. For example, you are very certain that as soon as your development has started for two days, you will have enough things to start testing.

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