Despite decades of debate on the subject, the Planning/Scheduling community remains deadlocked over what the terms Lead Values and Lag Values actually stand for. Some say “performance of work,” while others say “passage of time.” I happen to think that there will never be agreement until there is a rather dramatic paradigm shift – from “how it works” to “why we do it.”
This article offers a refreshingly unique approach to the matter, one that might not only resolve the dispute itself, but also offer a deeper understanding of what our Schedules are trying to say. But first, let me take a moment to clarify what we are talking about for the sake of those who are not familiar with the terms Leads and Lags.
Clarifying Some Basic Terminology
When two activities are scheduled to be performed at roughly the same time, with one starting or finishing slightly ahead of the other, the activities are said to be overlapped. In the Critical Path Method (CPM), the involved activities are known as Predecessor and Successor. In any of the illustrations in this paper, Activity A is the predecessor, and Activity B is the successor.
As shown in Figure 1, if the start of the successor activity is intended to start some time after the start of the predecessor, this is know in Dominant Project Management jargon as a Start-to-Start relationship. In Figure 2, we see that if the finish of the successor is intended to occur sometime after the finish of the predecessor, this is known as a Finish-to-Finish relationship.
Returning to the matter of prevailing terminology, there is confusion and disagreement in Project Time Management circles as to what the individual terms Lead and Lag actually stand for. Some believe that the term, Lead, refers to a Start-to-Start relationship and that a Lag is a Finish-to-Finish relationship.
But there are others who apply the terms Lead and Lag to the numeric values associated with Start-to-Start and Finish-to-Finish relationships. To explain, with any overlapped pair of activities we are concerned with two key pieces of information: what is being impacted, and by how much.
We have just explained the “what,” being either the start or finish of the successor activity. As for “how much,” this is the purpose of the numerical value. So, as shown in Figure 1, the SS:3 indicates that Activity B cannot start any earlier than three time units (usually, days) after the start of the predecessor, Activity A. Likewise and as shown in Figure 2, an FF:3 would indicate that the finish of Activity B cannot occur any earlier than three days after the finish of the predecessor, Activity A.
Thus, the SS or FF relationship types indicate what is being impacted, whereas the numerical values indicate how much of an impact to anticipate. While there is hardly agreement on the distinction between the terms Lead and Lead Value, or between Lag and Lag Value, Cognitive Project Management adopts the position that Lead and Lag refer to the what component (that is, either starts or finishes) while Lead Value and Lag Value refer to the how much component.
If all of this makes sense to you, then we may be ready to tackle the question posed by the title of this article. What I am asking is: what do the numerical values — associated with Leads and Lags – actually stand for?
To better appreciate the question, it might benefit us to consider a third common dependency type, where the predecessor and successor to do not overlap at all; rather, one activity follows the other. This is known as a Finish-to-Start dependency, and it states that the start of the successor is dependent on the prior finish of the predecessor.
As shown in Figure 3, the default Lead/Lag value is zero, meaning that just as soon as the predecessor finishes the successor can start. But, also as shown, one can assign a non-zero Lead/Lag value, such as FS:5 (middle panel), which would mean that the successor (Activity B) is intended to start no less than five days after the finish of the predecessor, Activity A.
Work Performance or Passage of Time?
Now that you have the terminology down, and you understand the question being posed by the article’s title, let’s get into it. Just what are the possible interpretations of Lead/Lag Values? There are two: Work Performance or Passage of Time.
Continuing with Figure 3 (middle panel), under the Work Performance interpretation, the SS:5 would be interpreted to mean that Activity B cannot start any earlier than five days after Activity A have been performed. Yet, under Passage of Time, it would be interpreted as meaning that Activity B cannot start until five days have transpired after the start of Activity A.
Passage of Time Example
To explain this distinction with an analogy, let’s say that you and a friend agree to meet for drinks after work; your friend works at a different location than you. You are 15 minutes from the tavern, which is 45 minutes from where your friend works. So you ask your friend to call you when she leaves work, and you plan to leave 30 minutes later. At 5:00 pm you receive a text message that she is pulling out of the company parking lot. Checking your watch, you make a mental note to leave your work at 5:30 pm.
At 5:45 pm you arrive at the tavern, but your friend is not there. So you call her and she explains that right after she texted you, her boss walked up to her car and began chatting with her about some work issues. Not wishing to be rude to her boss, she politely sat there while he rattled on and on about things that really could (and perhaps should) have been discussed the next day. Anyway, your friend tells you that she is still about five miles away.
Work Performance Example
The distance from your friend’s work to the tavern is fifteen miles, while yours is only five. Suppose instead of having her text you when she left work, you had instead asked her to text you when she was only five miles from the tavern. If she had, you would both have five miles to travel at that point, and would theoretically arrive at the same time.
But more to the point of this example, you would not have left at 5:30 pm, as you did. Instead, she would be calling or texting you at 5:45 pm and saying that only now was she five miles from the tavern.
Returning to the article’s question, when we see a Start-to-Start dependency with a three day Lead/Lag Value, as in Figure 4, what does that numeral “3″ stand for? Does it stand for a mere Passage of Time, or does it assume a certain amount of Work Performance? As I said at the outset, there is much debate over this seemingly simple question.
I won’t get into the various arguments in support of either side of the argument. If you are interested to form your own opinion, then I encourage you to Google on the terms and read for yourself. But let me tell you that the quagmire of conflicting thought goes far beyond what the Lead/Lag Values stands for.
It also involves a tangential question of whether Lead/Lag Values can ever be negative values, such as SS:-3. Check out the lower panel of Figure 3. We would interpret this dependency statement to mean that the successor, Activity B, is not expected to start any earlier than three days before the start of the predecessor, Activity A. If this sounds a little counter-intuitive … welcome to the craziness!
Cognitive Project Management’s Three Restriction Delay Stipulations
We thought about joining the debate(s), but decided that in the end little would be accomplished. For those who have an opinion, their passions would surely disallow them from easily changing their minds. For those who don’t have an opinion, they would be keeping themselves at arm’s length altogether.
So rather than extending the debate, Cognitive Project Management decided to simply take a position and give our reasons. We call these “Restriction Delay Stipulations,” in keeping with Cognitive Project Management’s alternative term for Leads/Lags; Restriction Linkages.
Note: The Start-to-Start dependency is called a Start Restriction; the Finish-to-Finish dependency is called a Finish Restriction; and the most common of all, the Finish-to-Start dependency, is called a Default Restriction. And the Lead/Lag Value is known as a Restriction Delay.
The following Restriction Delay Stipulations are not hard-and-fast rules, like the ones so often mandated throughout Dominant Project Management. Rather, they are merely recommendations (albeit strong ones) for those who choose to adopt the ICS-Compendium as a foundation for their own Project Time Management program.
Restriction Delay Stipulation #1: Concerning Restriction Delay in Start and Finish Restrictions
Restriction Delay Stipulation #1, which applies only to Start Restrictions and Finish Restrictions, says that whenever a Restriction Delay is expressed it represents a Work Performance period, and not the mere Passage of Time. Thus, a Start Restriction Delay quantifies the amount of the Restricting Activity’s (predecessor’s) Work Performance that must be completed before the Restricted Activity (successor) can start. Similarly, a Finish Restriction Delay quantifies the amount of the Restricted Activity’s final Work Performance that is being restricted, pending prior completion of the Restricting Activity.
Restriction Delay Stipulation #2: Concerning Negative Restriction Delays
Restriction Delay Stipulation #2 discourage, in the strongest possible terms, the use of negative Restriction Delays. In the case of Start Restrictions and Finish Restrictions which, pursuant to Stipulation #1 refer to Work Performance, a negative Restriction Delay would make no sense as it would be referring to the non-performance of Work. As for the Default Restriction which, according to Stipulation #3, refers to Passage of Time, a negative Restriction Delay would be calling for running the clock backwards.
Restriction Delay Stipulation #3: Concerning Restriction Delay in Default Restrictions
Whereas Restriction Delay values used in conjunction with Start Restrictions and Finish Restrictions are always reflections of Work Performance (per Cognitive Project Management), Restriction Delays are always interpreted to reflect the mere Passage of Time in Default Restrictions. A Default Restriction says that the Restricted Activity cannot start any earlier that X days after the Restricting Activity finishes, where X is a zero or positive value. Since the definition includes “after the Restricting Activity finishes,” there can be no work for the Restricted Activity to wait on, since the work of the Restricting Activity has already “finished!”
Three Restriction Delay Stipulations Both Logical and Constructive
Not only do the three Restriction Delay Stipulations make good common sense, they are also a welcome contribution to our efforts to bring clarity and order to the Project Time Management discipline. Once we accept that Lead/Lag Values in Start-to-Start and Finish-to-Finish dependencies refer only to Work Performance of the predecessor, and that in Finish-to-Start dependencies they refer to Passage of Time, debates about the meaning or reasonableness of negative Lead/Lag values instantly evaporate. Plus, for the first time we come to really understand and appreciate what the versatile Start-to-Start and Finish-to-Finish dependencies mean in practical terms.
So, the next time we are constructing a Project Schedule, and we are faced with assigning a Lead/Lag Value to a Start-to-Start dependency, we can stop to think about just how much Work Performance we want to see accomplished by the predecessor activity before we expect the successor to start. Likewise, with the Finish-to-Finish dependency.