When should you use Negative Lag? Are you looking for ways to fast track your schedule? Fast tracking the schedule, takes activities that are scheduled in series and performs all or some of their duration in parallel to shorten the project’s planned or remaining lifecycle. If you want to shorten your schedule by fast tracking then you most likely are considering the use of lags or negative lags, the latter otherwise known as ‘leads’.
The question we must answer is whether leads are a viable method of shortening the project?
Both lags and leads may have a significant impact on the length of the schedule. A lag is waiting time between the predecessor and successor activities. Typical lag applications includes Start-to-Start, Finish-to-Start, and Finish-to-Finish relationships. Leads are the amount of time that a second (successor) activity can start before the completion of the first (predecessor) activity. Leads are generally applied between activities in a Finish-to-Start relationship. The intention of leads is to put you ahead of schedule or to shorten the schedule.
However well intentioned your insertion of leads into the schedule; leads are, never-the-less, discouraged by most scheduling guidelines, in particular, the Defense Contract Management Agency (DCMA) 14 point assessment guidelines. The question remains what are the drawbacks to using leads and why do many expert practitioners discourage their use?
This article discusses the limitations of negative lag or lead in accurately modeling the parallel relationship between successor and predecessor activities.
Leads and Output
Leads are particularly useful for activities with long durations. The question to ask is whether a hard core FS relationship is required between two tasks or can the second task begin before the first task ends. A simple example is the installation of underground chilled water piping.
When, say only 7-days of trench excavation remains then the pipe fitters can begin installing pipe. The main issue in this case is to make certain that the output of the pipe fitter crew is less than the output of the excavation crew. A pipe fitter crew with a faster daily output would create the illogical situation where the second task in the FS relationship actually completes before the first. Of course, this is completely irrational and won’t happen. What will happen is that the pipe fitter crew will have to suspend work and have down time while trench excavation presses ahead. This then is a schedule resulting in an inefficient output.
Leads Predictive Nature
A lead or negative lag indicates that a successor activity can start a period of time before its predecessor activity is complete. In leads you are commencing one activity based upon the predicted end or completion of another activity. You, therefore, are leveraging the predicted completion of a predecessor scope of work to commence a successor activity.
The problem is that in reality it is very difficult to be able to predict the end of an activity or scope of work. And to link (or leverage) the start of another activity based on this end prediction is not good scheduling practice. Again, for the Finish-to-Start relationship with negative lag you are saying commence successor activity a period of time before the predecessor is 100% complete.
To clarify the problem look at it this way; you are linking and commencing activities based upon the future and predicted 100% completion of the predecessor. So you are deciding whether to start an activity now based upon a future activity situation, which is not ideal.
It is much simpler to understand a lag than a lead. It is easier to say commence lay pipe successor activity after 4-days of the predecessor excavate trench activity. To say, as in the case of leads, you will commence a successor activity an estimated 7-days before the completion of a predecessor activity is confusing. This is not to say that lags are without issues. The Start-to-Start with positive lag between predecessor and successor, in particular, has issues. To say that 4-days after the start of trench excavation the pipe fitters can commence installing the pipe is also accompanied with problems.
Your basis for commencing the pipe installation is not dependent on the completion of a scope of work, which is what you want. What if the excavation crew is not very productive, so after 4-days of excavation they have not made enough space for the pipe fitters to work at installing the piping? It is better to say once the excavators have completed a known scope of work, say the excavation of 100-ft of trench, then the pipe fitters may commence installing the piping. In this case excavate 100-ft of trench is the predecessor and the successors in parallel are excavate remaining 75-ft length of trench and install piping.
Lags can work well. The commonly used ‘concrete cure’ lag works well in a schedule because concrete takes a known amount of time to cure; and it is implemented after the completion of the pour concrete activity, a completed known scope of work. The concrete installation is a Finish-to-Start relationship with a positive lag. Again, it works well because the lag and successor activity are dependent on the completion of a known scope of work.
The Finish-to-Finish relationship with a positive lag also works well, because the lag and successor activity are dependent on the completion of a known scope of work. So create your schedule in such a way that your successor activity is linked to a completed known scope of work. Not the future estimated completion of a scope of work, but a completed scope of work.
It is simpler to understand lags, but both lags and leads may not be the best way to define the relationships between activities in a schedule. It is better for your successor activities to be based on the completion of a known scope of work.
Then you may proceed with working your activities in parallel to fast track your schedule. But to commence a successor activity after a period of time working on the predecessor will not guarantee enough progress has been made to successfully allow proceeding with the successor activity. And you certainly do not want to leverage your successor activity based upon an estimated future completion of predecessor scope of work. This does not provide the transparency that efficient schedules have.
If at all possible create your schedule such that successor activities are commenced based upon completed predecessor scope of work, such as in the case of a Finish-to-Start with positive lag or a Start-to-Start with positive lag. This will provide your schedule with the transparency and efficiency required for successful implementation. And you will avoid the inefficiencies of mobilized crew having to wait for predecessor activities to complete sufficient scope of work for the implementation of their successor activity.