Removing negative lag or leads from your schedule is a noble cause. But replacing negative lag with positive lag and, perhaps, a start to start (SS) relationship may only be a partial solution.
Negative lag or (lead) is a real problem in schedules. The following blog The Negatives of Negative Lag explains in detail the negatives of negative lag.
Possibly, the main issue with negative lag is its predictive nature or dependence on prognosticating the future. While that is kind of the point when doing schedules, it should be done based upon a sound logical work sequence, not an unqualified assumption of actual work sequence.
So, in negative lag you are typically linking and commencing a successor activity based upon the future and predicted 100% completion of the predecessor. So you are starting a successor activity early based upon a future activity situation, which is problematic.
Leads are to be avoided, and the defense contract management agency (DCMA) 14-point assessment forbids leads in scheduling. A scheduling solution acceptable to DCMA is to replace the negative lag with a SS relationship modified by a positive lag. This solution, though acceptable, only takes you partway to where you want to be on your scheduling journey.
This article demonstrates why finish to start (FS) relationships and no lag is a better leads scheduling alternative than the SS relationship and positive lag.
If your intention is to replace a SS and positive lag with FS relationships you are in good company; the DCMA relationships assessment prefers FS relationships. Later we’ll explain why. But let’s begin our demonstration with a lead, Figure 1.
This lead modified relationship says seven days before you ‘receive tiles onsite’ you commence mobilization. This is a nice fit for our activities, and appears to describe the true narrative of the project situation. The problem with this logic, again, is that you are commencing mobilization based on the future receipt of the tiles. What if there are manufacturing problems or other delivery delays, so that the tiles do not arrive on time and as scheduled? Oops! Well, you then have to delay construction and your crew will have to remain in a mobilized state until the tiles finally arrive. Not good!
An acceptable solution to this leads dilemma, Figure 2, is to link mobilization to the preceding manufacturing activity in a SS relationship and positive lag.
The positive lag modified SS relationship reads eight days after you commence manufacturing you can begin mobilization and mobilization continues until receipt of the tiles on site. This is clearer and much easier to understand logic than the leads modified schedule. But it is not a perfect solution; the logic has its own issues.
Yes, to say that eight days after the start of manufacturing we can begin mobilization on site has its own logical problems. Your basis for commencing mobilization is not dependent on the completion of a known scope of work, which is what you prefer.
What happens if the manufacturing crew is not very productive, so after eight days of manufacturing they have not manufactured enough parts to warrant the commencement of mobilization? Additionally, our logic does not provide the impetus for the start of mobilization. It is better say after the manufacturing crew has completed a known scope of work mobilization may proceed.
To describe this known scope of work you would most likely break up the manufacturing task and connect the resulting elements with FS relationships, hence a reason the DCMA prefers FS relationships. In Figure 3 we have our schedule logic that breaks up manufacturing into two known scopes of work: ‘manufacturing sail tiles’ and ‘manufacturing main hull tiles’.
The driver for the start of mobilization is the completion of the submarine scale model sail tiles. We now know that the 8-day lag between the start of manufacture and start of mobilize is really the effort to manufacture solely the sail tiles. This is a small portion or phase of the whole model submarine tile manufacturing effort, and may serve as a quality assurance check point before continuing production with the main hull tiles. The sail tiles may also be separated out because these tiles have a unique shape, and slightly different production process.
Regardless, we have a more insightful narrative of the project schedule. This more detailed project story was achieved, again, by breaking up tasks into smaller elements and connecting the resultant component parts with FS relationships.
Note that Figure 3 displays only a portion of the schedule for simplicity. These tasks will be followed by installation tasks for both the sail tiles and main hull tiles. Also, manufacturing tasks can be further divided into manufacturing, quality assurance, and delivery tasks for both the sail tiles and the main hull tiles.
The main point is that separating the manufacturing task into ‘manufacturing sail tiles’ and manufacturing main hull tiles’ links the mobilize tasks to a known scope of work, which is a more descriptive solution to the negative lags dilemma.
Negative lag or leads are definitely to be avoided and are not allowed by the DCMA scheduling guidelines. An acceptable and, perhaps, popular alternative to leads is the insertion of a SS relationship modified by a positive lag.
This works and is agreeable, but is not the best solution. You really want your successor task initiation to be based upon a completed known scope of work. This tells you the impetus for the start of the successor activity.
So break up the tasks into smaller components and connect the resulting parts using FS relationships. This provides better schedule narrative clarity, and saves you from having to explain your positive lag to stakeholders.