The Naval Facilities Engineering Command’s (NAVFAC) Initial Project Schedule (IPS) checklist item 64 says finish to start (FS) relationships should have zero lag. What is the reasoning for this stipulation?
Scheduling guidelines are good to study because understanding the principles behind the guidelines increases one’s scheduling insight, which helps the creation of high quality schedules. In a previous blog I stepped through and explained several rules in the NAVFAC IPS checklist. Refer to the following blog A Short Walk through the NAVFAC IPS Checklist.
To be more specific, in the schedule logic category I explained in detail why there should be no negative lags in the schedule. However, I was silent on another rule that appears in the schedule logic category. This is item 64 “Finish-To-Start relationships are assigned 0 Lag”. Recently, I came across an explanation for this rule that compels me to share the reasoning behind this requirement. Consider this blog as putting together another piece of the puzzle that when complete will contribute to a portrait of a high quality schedule.
This article explains the reasoning behind the NAVAC item 64 requirement that FS relationships should have zero lag.
Well, lags even positive lags should be avoided in scheduling. This is explained in the following blog titled The Negatives of Positive Lag.
In short, positive lags should be avoided because of their lack of visibility. Lags, both positive and negative, appear as lines on the Gantt chart and, therefore, are not very apparent. In the mentioned “The Negatives of Positive Lag” blog I propose shorter known-scopes-of-work tasks separated by FS relationships, as a better alternative to positive lags.
So where possible, shorter well-defined tasks that have FS relationships are preferred. In fact the Defense Contract Management Agency (DCMA) relationships assessment in their 14-point Assessment guidelines says that FS relationships should comprise 90% of the schedule relationship types. So start to start (SS) and finish to finish (FF) relationships should have limited usage: no more than ten percent of activity relationships. Both the DCMA and NAVFAC guidelines are also adamant that negative lags are not allowed.
Okay, it is clear; negative lags are not allowed. However, what do the guidelines say about positive lags and associated FS, SS, and FF relationships? Well, the DCMA 14-point Assessment says SS and FF positive lag modified relationships can have limited use, as previously mentioned. This brings us then to the topic of discussion; positive lags modifying FS relationships. The NAVFAC guideline item 64 specifically says to assign FS relationships 0 lag. This seems to be a roundabout way of saying that FS relationships should not have any lag, either positive or negative.
The question at hand is why this limitation on the assignment of positive lag to customize FS relationships? Why should FS relationships have zero lag? The reason for this NAVFAC requirement is that positive lags on FS relationships appear as time gaps on the Gantt chart. SS and FF relationships with small positive lags will not appear on the schedule as if no work is progressing during their scheduled period of time. In Figure 1 we have a concrete floor installation that has a positive lag between pour concrete and strike forms.
This Gantt chart may confuse some stakeholders not familiar with the project. It looks like nothing is happening the second week of work. You, however, are aware that the concrete is curing during this apparent gap in time. A clearer schedule would model the concrete curing as a separate task with no resource loading.
Now let’s look at a SS and positive lag schedule for the installation of underground piping, Figure 2.
Again, this schedule is not ideal as the positive lag is not well documented. The reason for the positive lag is to defer the pipe installation a few days until enough trench is excavated to provide room for the pipe installation crew. So the delay supports the pipe installation crew’s required workspace. This offset SS relationship modifier appears on the Gantt chart as a line, which is not very descriptive.
Never-the-less, the positive lag modified SS or FF works, because there are no apparent time gaps in the schedule where it looks like absolutely nothing is happening. Thus, limited use of SS and FF positive lag modified relationships is acceptable, but FS relationships should not have any lag to avoid the appearance of Gantt chart time gaps.
One more reason for the restrictions on lag in schedules is that they are open to abuse. Some schedulers may use lags to force dates onto successor activities that would otherwise be earlier than desired. This would again leave inexplicable gaps in the schedule logic as I previously mentioned, but for far less defendable and possibly nefarious reasons.
At the end of the day it all comes down to making sure the schedule dates are driven by logic, not forced by the will of the scheduler. Removing lags from the scheduler’s toolbox helps prevent any such misgivings, accidental or otherwise.
SS and positive lag or FF and positive lag are more agreeable to the NAVFAC check list, because they do not introduce apparent discontinuities in work. Less informed stakeholders will be able to observe that construction is continuous through the associated tasks or, perhaps, for the life of the project.
The concrete floor installation construction (modeled using an FS relationship and a positive lag modifier) appears to be intermittent, which may confuse less informed stakeholders. Therefore to avoid the appearance of intermittent construction work, scheduling best practice guidelines from the NAVFAC check list say that FS relationships should have zero days lag.
The preference is to model positive lag in our concrete floor installation as a separate task. Yes, a high degree of customization is possible in defining and modifying schedule relationships, but best practice guidelines call for caution when implementing lags. Again, the goal is to present the schedule in a way that stakeholders clearly understand and can therefore believe.