The Applications of Negative Lags

Negative lag, or lead, is a scheduling term that allows the successor activity to start before the predecessor activity finishes. Essentially, it creates an overlap between the two activities.

For example:

• If Activity A has a Finish to Start (FS) relationship with Activity B and a negative lag of -2 days, Activity B can start 2 days before Activity A is finished.

Negative lag indicates that a successor activity can start a period before its predecessor activity is complete. In leads, you are linking one activity based on the predicted end or completion of another activity. The problem is that it is very difficult to be able to predict the end of an activity or scope of work. And to link the start of another activity based on this end prediction is not good scheduling practice.

Why is Negative Lag generally not recommended in scheduling?

1. Unrealistic Scheduling Assumptions

Negative lag assumes that the successor activity can begin before the predecessor is complete. This can be misleading because it may not accurately reflect the real-world dependencies and constraints of the project.

2. Lack of Clarity

Negative lag can make schedules harder to understand. It creates confusion, especially for stakeholders who might see the overlap as a mistake or poor planning.

3. Difficulties in Progress Tracking

Tracking progress becomes challenging with negative lag. It can hide the actual start and finish dates of activities (Start & Finish dates aren’t clear, ex: When will activity B start? It starts 2 days before A ends), making it harder to monitor the sequence of tasks and identify delays.

Example: Wall Painting and Wall Plastering

Normally, in a construction project:

Wall Plastering (Activity A): This activity involves preparing and plastering the walls.

Wall Painting (Activity B): This activity begins after the plaster is fully dried and completed.

In most cases, Wall Painting would start after Wall Plastering is done, with some time added for drying (this is a typical “lag” or delay between activities).

Example of Negative Lag:

Now, imagine the schedule says:

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Wall Painting (Activity B) will start 2 days before Wall Plastering (Activity A) finishes.

This means the painters will start painting the lower parts of the wall, even though the plastering on the upper walls isn’t done yet.

Problems with Negative Lag:

Tracking Issues: If Wall Painting starts before Wall Plastering finishes, it’s hard to figure out the real completion dates for each task. For example:

When did painting start?

Did the plastering finish on time, or did it delay the painting?

Coordination Issues: Workers from different teams might overlap, causing disruptions. The painters could struggle if the plaster isn’t fully ready.

Delays Get Hidden: If plastering gets delayed but painting starts anyway (due to negative lag), the delay might not appear clearly in the schedule, making it harder to address.

Why Avoid Negative Lag:

Using negative lag in scheduling makes it tricky to see clear task sequences and identify where delays happen. Instead, it’s better to use logical relationships where tasks follow each other naturally, like:

Plastering ends → Drying time → Painting starts (with a positive lag for drying).

This approach ensures better clarity and tracking in your project schedule.

4. Conflict with Best Practices

Scheduling standards such as those outlined by the Project Management Institute (PMI)

DCMA (Defense Contract Management Agency) advises against negative lag and recommends showing overlaps with Activities (sometimes called “Buffer” or “Dummy” activities, they just exist to hold and represent the delay/lag duration).

5. Risk of Miscommunication

Negative lag can lead to misunderstandings among team members, contractors, and stakeholders. It might be unclear why or how the overlap occurs, leading to confusion during execution.

Recommended Options:

Use Explicit Overlap Activities:

Instead of using negative lag, define a specific Activity that overlaps the two activities. For instance, introduce a “preliminary work” activity to start during the predecessor’s execution.

Adjust Durations or Add Relationships:

Modify the activity durations or use proper Start-to-Start (SS) or Finish-to-Finish (FF) relationships to reflect the overlap realistically.

Include Milestones:

Use milestones to clarify dependencies and represent critical points in the overlap.

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