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27 January 2011

Fine-tuning Task Relationships with Lead and Lag

David A. Zimmer Print

PMBOK® Definition:

Lag: A modification of a logical relationship that directs a delay in the successor activity. For example, in a finish-to-start dependency with a ten-day lag, the successor activity cannot start until ten days after the predecessor activity finishes. See also lead.

Lead: A modification of a logical relationship that allows an acceleration of the successor activity. For example, in a finish-to-start dependency with a ten-day lead, the successor activity can start ten days before the predecessor activity has finished. A negative lead is equivalent to a positive lag. See also lag.

Practical Definition:

Lag: A pre-defined delay between the predecessor and successor activities, as in, the delay caused by paint drying before picture hanging.

Lead: A pre-defined acceleration of the successor activity in relationship to the predecessor relationship. Also known as negative lag. Used to create overlap of tasks to shorten the overall length of the project.

Understanding Lead and Lag

Lead and lag time are used in project schedules to fine-tune relationships between predecessors and successors. Many people, especially laymen – those who talk about projects, sling project management terms around, but don’t fully understand the true meaning behind the terms – confuse these terms with slack or float. They are completely different.

Slack and float mean the same thing, as described in “What is Slack? What is Float? Is There A Difference?” They are mathematical calculations and change over the course of the project. They are a result of the planning process and exist on the non-critical path only. Lead and lag can exist in both the critical and non-critical paths.

Lead and Lag are defined during the planning stage and do not change over the course of the project, unless explicitly changed by the project team.

Lag in a Finish-to-Start Relationship

Lags are used to wait a period of time between tasks. For example, concrete must cure before it can be used. Therefore, the builder pours the concrete, waits four days and then builds the walls on the concrete. The Gantt chart below shows a lag between pouring the concrete and building the walls using a finish-to-start relationship.
Lag in a Start-to-Start Relationship

Let’s see what a lag in a start-to-start relationship might look like. Using the example of market research as described in the article, “Four Logical Relationships of Project Management: What They Are and How To Use Them,” we developed a survey, distributed the survey, received responses and then tabulated the results. We stated “Receive Survey Responses” and “Tabulate Responses” as start-to-start relationship. In reality, we’d wait a few days before tabulating the results. So, the resulting Gantt chart would be
Lag in a Finish-to-Finish Relationship

Using the cooking example from the Four Logical Relationship article, let’s add a pie to the menu. We can use a finish-to-finish relationship so the pie finishes 15 minutes after the food is finished. Here is the new Gantt chart:
Lag in a Start-to-Finish Relationship

Using our dinner boat cruise example from the same article, there is a 30 minute overlap between ticket window closing and the boat leaving the dock so we can pre-sell tickets for the next day. Here’s the Gantt chart:
Lead – Accelerating Successor Activity

Defining lead time into the plan accelerates the successor activity’s start time. We use lead time to shorten overall project schedules or a particular portion of the schedule. When fast-tracking a project, that is, accelerating various tasks to shorten the project, we use lead time to overlap activities.

Lead Time in a Finish-to-Start Relationship

Using the car washing example from our article mentioned above, let’s pretend we only have 45 minutes for a job taking normally 60 minutes. Normally, we’d spend 15 minutes washing, 15 minutes drying and 30 minutes waxing the car. We will overlap the washing and drying by 7.5 minutes and overlap the drying and waxing by another 7.5 minutes. The resulting Gantt chart looks like:
Of course, by overlapping the tasks, or fast-tracking them, we increase the risk of having to re-do some work. In the case where the tasks are ordered sequentially, we know the final result of the predecessor task before the successor starts. When fast-tracking, the predecessor may finish slightly differently than expected, or in the case of the car washing example, overspray from rinsing one portion of the car will require us to re-dry because of the overspray.

Lead Time in Start-to-Start Relationships

Using lead time in a start-to-start relationship causes the successor activity to start before the predecessor task. An example might be both the electrical work and plumbing can start at the same time in a house construction. Although this example might be a bit of a stretch, we show it here for illustrative purposes. Let’s say the electrician can start 30 minutes prior to the plumber. The Gantt chart would look like
Lead Time in a Finish-to-Finish Relationship

Going back to our “Four Logical Relationships of Project Management” article, we used meal preparation to illustrate a finish-to-finish relationship, where we wanted all the food to be ready at the same time. Actually, meat should “coast” for 15 minutes before cutting, which means we want to pull it out of the oven 15 minutes before serving. Therefore, we would construct our Gantt chart as follows:
We see, by adding in the lead time, we should put the meat and the potatoes into the oven at the same time. The vegetables and pie would go in 45 minutes later, at the same time we pull the meat out to let it coast. By following this timing, the main course of the meal is ready to eat at the same time and the pie is done when we finish eating the dinner.

Lead Time in a Start-to-Finish Relationship

Once again, using the same example as before – the dinner boat cruise – we can add in lead time to account for the time the owner would close the ticket sales and then hurry to the boat to collect the tickets from the boarding passengers. Let’s say, the owner closes the window 15 minutes before the boat’s departure from the dock. Using the start-to-finish relationship in case the boat’s departure is delayed, we inject the lead time designation. The resulting Gantt chart looks like
Conclusion

Lead and lag permit us to fine-tune our project plan. A lag is a pre-defined delay between the predecessor and successor activity. A lead is a pre-defined acceleration of the successor task.

We use lags when a delay must occur before the successor activity can proceed, as in waiting for the paint to dry or concrete to cure. We use lead times to compress the project schedule as in fast-tracking a project or when we can overlap tasks.

Lead and lag times can be placed in any of the four relationships of project management.

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