Removing Negative Lags from Primavera P6 Schedules

Some industries frown upon the use of negative lags or leads in a project schedule. For example, on some government contracts you may find language that prohibits negative lags or leads. In these cases, you often have little choice but to remove the leads from your schedule when putting together your baseline. But it just seems that your leads are a perfect fit for the activity relationships you are trying to describe. What do you do?

To clarify, lag is waiting time between the completion of a predecessor activity and the start of its successor activity. Negative lag, (a.k.a. lead) is the amount of time that a successor activity can start before the completion of a predecessor activity. Negative lag is discouraged by most scheduling guidelines. “The Negatives of Negative Lag” for a comprehensive discussion on why negative lag is discouraged. You will also find that the DCMA’s 14 point assessment ‘check # 2 – Leads’ will mark the presence of even one lead as a fail for the schedule.

Leads are typically used in Finish-to-Start (FS) activity relationships, Figure 1, to describe the start of a successor activity before the finish of a predecessor activity.

Figure 1

In these cases the FS dependency between the two activities is not “hard”, so you can actually commence the successor a period of time before the completion of the predecessor. However, leads are confusing and discouraged, and alternatives exist for scheduling the project without leads.

This article demonstrates a typical use of leads in schedules and how to model alternative relationships to remove the leads and maintain an appropriate description of the work.

Please note that this is just one suggested technique and example for lead removal. Ultimately every situation and schedule is unique and it will be your best judgement as to how to model logic in alternative ways to avoid using leads.

In Figure 2 we have a Manufacture and Installation project schedule.

Figure 2

This schedule uses a lead to describe the FS relationship between ‘Receive Equipment on Site’ (predecessor) and ‘Mobilize on Site’ (successor), Figure 3.

Figure 3

The Figure 2 bottom details shows that seven days before ‘Receive Equipment on Site’ is complete you start to ‘Mobilize [Resources] on Site’. So mobilization starts seven days before the estimated delivery of equipment. That seems straight forward. Once your delivery date is set you schedule mobilization to occur seven days before. This seems to be a nice fit for the FS relationship with negative lag.

The question is how can you replace the FS plus lead with logic that still describes the proper relationships between the mobilization and delivery equipment received activities? The answer is the schedule displayed in Figure 3.

Figure 4

In Figure 4, we simply replace the FS and negative lag relationship with a Start-to-Start (SS) and positive lag (8-days) relationship between manufacturing and mobilization activities. (See SS + 8D Lag relationship in Figure 5.)

Figure 5

We can also include a Finish-to-Finish (FF) relationship between mobilization and delivery, i.e. ‘Receive Equipment on Site’, Figure 6.

Figure 6

(Note, a minor point is that you may want to make ‘Receive Equipment on Site’ the predecessor and ‘Mobilize on Site’ the successor. It depends whether the mobilization effort completion is dependent on first receive delivery effort completion. Again, it is a minor point.)

Why is the SS w/ positive lag better? For starters you removed the lead, so your schedule meets the no leads requirement. It also is less confusing. Here you are saying that 8-days into manufacturing you proceed with mobilization, and mobilization completes just as the equipment arrives.

If you use the FS with lead you will say 7-days before the 100% completion of delivery you may commence mobilization. Here you are always looking toward the predicted completion of delivery to adjust your mobilization effort. But manufacturing (and QA) progress significantly affects the delivery date, so why not base your mobilization commencement on completed manufacturing scope of work?

This is what the SS w/ positive lag does. This schedule anticipates and plans that after 8-days of manufacturing enough manufacturing scope will be complete to begin commencement of mobilization. After the 8-day (mobilization) lag or 8-days of manufacturing, you examine your manufacturing progress. If manufacturing is on schedule you proceed with mobilization. So you are basing the commencement of mobilization at the point where mobilization is scheduled to occur on completed manufacturing scope work, and not on the future predicted completion of delivery.

How certain is that delivery date? In both schedules mobilization occurs before ‘Quality Acceptance Testing’, so both scenarios risk the possibility of parts not passing Quality Assurance (QA), which will increase the duration of the manufacturing activity. And delay the delivery date. The delivery date is not certain.

In the schedule without leads, again, we start mobilization before ‘Quality Assurance Testing’, which is a risk. We measure manufacturing progress and commence mobilization efforts once sufficient manufacturing scope has been completed. Here we are simply saying that 8-days into manufacturing we plan to have made enough manufacturing progress to proceed with mobilization in preparation for parts delivery.

This is a softer and less rigid prediction based on actual progress, and not on a future activity situation. We can also examine the completed (8-day) effort and see exactly what scope of work was accomplished before giving the go signal to proceed with mobilization. So if it doesn’t look like we’ve made enough manufacturing progress we can simply delay the mobilization activity by extending its lag.

In the schedule with leads, the required completion amount of manufacturing scope of work before mobilization is not specified, and is, therefore, unclear. There is no link between manufacturing and mobilize to clarify their relationship. Your mobilization effort will therefore commence without confirmation of the necessary completed manufacturing scope of work.

Of course, you could include the required manufacturing progress before mobilization commencement (SS plus positive lag) in the schedule with leads, but then your FS and negative lag relationship would be redundant.

In the leads schedule, as mentioned, we predict the future delivery date and then work backwards to determine the commencement of mobilization. This is confusing. We are not basing mobilization on progress or logic, but on a prediction. The start of mobilization is therefore based on the predicted future completion of manufacturing, QA, and delivery.

Again, it is difficult to try and determine exactly when in your schedule you are expected to be 7-days away from the completion of manufacturing and delivery. In all likelihood, our mobilization activity will begin before we realize that our estimated delivery date was too optimistic. But it is too late, we have already begun mobilization.

Summary

A common scheduling practice places commencement of mobilization before parts delivery, so installation efforts can proceed immediately after delivery. Schedules that place mobilization activities before quality assurance are taking a risk. However, the schedules without leads link mobilization directly to manufacturing and base the commencement of mobilization on completed manufacturing effort, and are more reliable. You can pause to examine what your manufacturing effort actually accomplished (in scope), and adjust the start of mobilization, accordingly.

Schedules with leads base the start of mobilization on the predicted future “hard” delivery date, and are more complex and less trustworthy. You must accurately predict the future delivery date and work backwards from there.

The effort to simplify schedule logic is the main reason that most scheduling guidelines discourage the use of leads. The replacement of a FS and negative lag with a SS and positive lag is not a perfect solution, but, again, is less confusing. And it will enable your schedule to meet the no leads requirement.