I’ve heard from some planning consultants that negative lags (leads) must be prohibited and replaced with positive lags from a predecessor activity. While this workaround has certain benefits it is not the universal way for managing activity overlaps. It’s essential to also consider the downsides as well.

In general, activity overlaps are beneficial as they accelerate project delivery. However, creating and managing overlaps increases the delivery model’s complexity. Projects can be delivered quicker and with less funds if the complexity can be effectively managed.

There are three possible ways to simulate activity overlaps in a project delivery model:

  • FSNL: Use Finish-to-Start dependencies with negative lags (leads)
  • SSPL: Use Start-to-Start dependencies with a positive lag
  • Apply artificial split of the predecessor activity

Each option carries benefits and downsides.

In this post Artificial activity split problem. Resource levelling challenge, we reviewed the downsides of an artificial split. Now, let’s discuss SS dependencies with a positive lag option.

In this example ‘Activity D’ could/should/must start 1 day before the milestone.

Currently, the milestone date is driven by the completion of Activity C. So, replacing the FS-1d dependency with SS+3d lag is possible. Each activity and milestone’s start and end dates will be the same.

The SSPL workaround addresses some problems but also creates other planning simulation issues.

Advantages

There are reasons why planners may want to use SSPL instead of SSNL.

Benefit 1
For some people, it is easier to imagine a delay than overlap.

Benefit 2
Negative lags can be used to hide project delays, and it is easy to prohibit them completely rather than to identify such cases.

Benefit 3.
Situations when the start of the successor’s activity is before the start of the predecessor’s activity look like anomalies.

The start date of Activity D is before the start date of the Milestone. Practically, there is no anomaly in this example as the milestone is not an activity (just a logical connector), and the start date of Activity D is later than the start dates of all predecessor activities (A, B, and C).

However, the logical anomaly is possible in other scenarios and must be addressed.

Benefit 4.
An overlap may be linked to achieving the volume from the predecessor activity.
For example, when 80% of volume activity C is achieved, the successor can start. In this case, the SS + lag dependency is the correct representation of logic. However, to simulate it correctly, activity and lag need to be volume-based, not duration-based. 80% of duration may not be the same as 80% of volume.

Unfortunately, many popular planning tools don’t support volume lag. Read more in the post: Volume lags

Disadvantages

The SSPL approach has downsides.

Problem N1
The duration of positive lag (SS+ 3d) has to be aligned with the predecessor activity duration (4 days). There are MANY reasons why the duration of Activity C may change: change in activity or resource calendars, change in resource assignment, clarified volume of work, etc. Regardless of the change, the duration of the lag must be revised and updated. It may not be easy to identify which schedule changes impact lag duration.

Problem N2
The Milestone has non-driven predecessor activities (A and B). If any of them are delayed and push the milestone, it should also push the start of Activity D. However, as it is linked to Activity C only, the schedule incorrectly shows that Activity D can start as planned. As a workaround Activity D may have SS + lag logic to each predecessor activity but it even further increases complexity due to the problem N1.

Problem N3
If Activity C has a delay after it commences, it will push the milestone but not Activity D.

Problem N4
The SS + lag logic actually doesn’t address Benefit N2. Projects can still hide delays with the SS + lag by not updating the duration of lag correctly.

Problem N5
This workaround only works if the lag calendar is the same as the predecessor calendar. Popular planning tools have limited capability to configure lag calendars. Microsoft Project applies successor calendar. Primavera allows configuring the lag calendar, but configuration can only be done for the whole project, and it could be scenarios when it lag calendar need to be associated with successor duration, not predecessor.

Problem N6
The above problems may impact the result of the Monte Carlo Simulation Analysis. Read more on the post: Artificial activity split problem. Resource levelling challenge.

Summary

Activity overlaps are beneficial but must be correctly simulated in the project delivery model.

Application of the ‘SS + lag’ logic is relevant for some scenarios, but it is not a universal way to represent activity overlaps. This approach has downsides that downgrade potential benefits.

Alex Lyaschenko

PMO | Portfolio Planning & Delivery | PMP | P3O Practitioner | AgilePM Practitioner | Six Sigma