Negative lags are not recommended by the Defense Contract Management Agency (DCMA) 14-point assessment because at best they can interrupt the forward flow of activities from beginning to end. And, if used improperly, negative lags can easily generate unrealistic network logic.
This article examines why the DCMA lead criteria forbids the use of negative lag and looks at possible alternative scheduling approaches. Potential resource conflicts due to leads, however, is not addressed.
First, let’s be clear about one thing – this article is talking specifically about the DCMA 14 point assessment which has a zero tolerance for negative lag in any defense schedule it is auditing. If you use negative lag, no matter what the rationale for doing so, your schedule will fail the assessment. However, use of negative lag in civil/non-defense contracts is performed at the discretion of the scheduler. And, while generally not recommended for the reasons spelled out in this article, is not an absolute rule – just a guideline.
Generally speaking, the DCMA 14-point assessment measures schedule quality, and alerts schedulers when the schedule has issues that may jeopardize the projects opportunity for success. The 14-point assessment seal of approval provides confidence that the schedule is a well-modeled and therefore practical schedule. You can audit your own schedule using tools such as Steelray to see if you will pass a DSCM 14 point assessment.
The second check or criteria in the 14-point assessment looks for negative lag also known as a ‘lead’.
Many schedulers prefer the use of negative lag in their schedules because it helps to compress the duration of the project. Some schedulers also say insertion of negative lag helps to better describe the true narrative of activities. The DCMA negative lag guidelines, however, forbid the use of any negative lag in the schedule. As previously mentioned, any schedule that has negative lag fails this second check in the 14-point assessment. So if you are working on a defense contract that the DCMA are auditing – don’t use it.
Let us first introduce the precedence diagram and associated positive or negative lag relationship modifiers. The precedence diagram, Figure 1, describes the cause-and-effect dependencies between two tasks.
A dependency is the relationship between the (start or) finish of one task and the start (or finish) of another task. In this diagram, Figure 1, task A is the independent cause or driver and task B is the dependent effect or driven. There are four relationships one may apply to model the interaction between tasks. Refer to the following blog titles The Cause and Effect Task Dependency Paradigm for a primer on the four relationship types.
The most common relationship and the one most often used with negative lag is the Finish-to-Start (FS) relationship.
The relationship dependency between two tasks in the precedence diagram may be modified by adding waiting time or delay time. This waiting time modifier is referred to as lag. Figure 2 displays a FS relationship and positive lag modifier. In this case the lag is 5-days. Here you must wait 5-days after the completion of task A before you can begin task B.
Lag modifiers come in two flavors positive lag and negative lag (or lead). Lead is negative time, which sounds a bit confusing. In Figure 3 we have our classic FS relationship modified by a negative lag (or lead). This situation reads “7-days before the finish of activity A you may proceed with activity B”.
In this modified relationship you are saying that the FS relationship between activity A and activity B is not a hard set in stone relationship. The FS relationship in the precedence diagram says activity A must be completely done before commencing activity B. Well, the negative lag says that is not necessarily true. We can, perhaps, commence activity B a few days before the actual completion of activity A.
Schedulers like lead as it is a way to fast track the schedule by performing a portion of each activity in parallel. And sometimes a lead modifier seems to better fit the activity narrative. For instance the schedule that says 5-days before you receive equipment on site you begin mobilization. That appears logical. You know the delivery date, so 5-days before that date you mobilize your material, and then crew. It’s a nice fit between mobilize and receive equipment on site.
But what happens if your delivery date is delayed? Oops! In that case you have your crew ready to proceed, but they are left waiting in limbo for the equipment to arrive on site. Not good! Thus the true story of negative lag is revealed. Negative lags imply the unusual measurement of negative time and require an exact foresight about future events.
Besides the required future foresight fundamental flaw with negative lag the DCMA 14-point assessment says that leads may distort total float and adversely affect the critical path. The DCMA assessment does not provide any examples of float distortion.
However, in Figure 4, we have a clear improper application of negative lag that not only distorts float, but violates network logic.
In Figure 4, the lead spread is so wide that the successor activity C actually finishes before the predecessor activity B. This is a clear violation of network logic. Why define a FS relationship between activities B and C, if you let the schedule dishonor it.
Note that activity C and activity ‘project finish’ both have a total float of 2-days, which means they can both be delayed 2-days without delaying the entire project. Well, ‘project finish’ is the end of the project, so these two activities, C and milestone ‘project finish’ should have 0-days total float.
Also note, activity B extends beyond the project finish date, but has 0-days total float when logic would say it should have minus 2-days total float. Note further that activity C is not on the critical path, so we are additionally having issues with our critical path.
What’s happening here is the software says the end of the project is 19-Jan-2018, because activity B extends till that date. Watch what happens when we add a 17-Jan-2018 ‘must finish by’ project constraint, Figure 5.
Activity B now correctly has minus 2-days total float. So with the addition of the project constraint we know our schedule has a problem; activity B’s minus 2-days total float is not good.
Another issue the 14-assessment addresses is the clarity of the schedule. In many cases schedules with leads indicate that the schedule does not contain a sufficient level of detail. Exactly what is the trigger in the FS relationship with negative lag to motivate the commencement of the successor activity before the completion of the predecessor? The narrative of negative lag is better represented by either Start-to-Start (SS) relationships and a positive lag or FS relationships of shorter duration and no lags.
Let’s demonstrate FS relationships of shorter duration and no leads. First let’s view a construction project that includes lead, Figure 6.
In this schedule 2-days before you finish installing the wall frame you may commence installing drywall. The question is what is the trigger that allows commencement of install drywall? As you can see, the lead does not provide enough insight. It does not provide the compelling reason drywall installation can proceed, other than 2-days wall framing remaining. In Figure 7 we have the same project and a more descriptive schedule.
In place of the lead we have shorter FS relationships and no positive or negative lag. Not only does it tell us we can commence drywall after 2-days of wall framing, but we also know the reason. Wall framing the north side of the construction site is complete. After wall framing the north side, we know there is enough framing complete to commence drywall installation. And installation of wall framing continues in parallel with the drywall installation.
The updated schedule tells the construction crew to continue wall framing, but on the south side, and wall framing the south side must finish before we finish drywall installation. The second schedule, having shorter durations and no lead, provides the analyst reviewing the schedule with more and sufficient detail to understand the true narrative of the schedule.
Despite their support of schedule compression efforts and possibly a nice fit with the schedule narrative, leads are never-the-less forbidden by the DCMA 14-point assessment.
The problem with leads is that they disrupt the forward flow of the schedule at best, and at worse may violate network logic. Leads also indicate that, perhaps, the schedule does not provide sufficient detail. A better alternative are shorter FS relationships and no lag, where the trigger for the succeeding activities is clearly defined.
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