What Lead/Lag Values Mean

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.

P04-01, SS

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.

P04-02, FF

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.

P04-03, FS

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.

P04-04, Work Performed

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.

9 Comments to What Lead/Lag Values Mean

  1. Zach Reed says:

    Responding to the comments below, I have been advised by two highly experienced Project Controls practitioners who take separate stances on this issue. One suggests the use of restriction delays indicating that the details can be enumerated in the software, and that is how the delays are tracked on a monthly basis. The other suggests that restriction delays should not be used, and instead separate activities be inserted into the schedule to account for the delay. Personally, I prefer the latter. This way is clearer as to why the delay is there in the first place. It is also transparent in showing that there is a delay present in the first place. Speaking from my experience with Primavera P6, there is no clear indication on the activity if a restriction delay has been applied or not. So, I plead for the sake of clarity, that using separate “place-holder” activity is a better option.

  2. Sue Backiel says:

    The example of the friends meeting for drinks is a great case in point as to why “work performance” should be used when dealing with Start Restrictions and Finish Restrictions and using “passage of time” when using Default Restrictions. These stipulations clarify and simplify the lead/lag controversy.

  3. Murray Woolf says:

    Dave,

    Thank you for your thoughtful comments. In response to your points about Stipulation #1, allow me to emphasize the word “stipulation.” I used this term because I wanted to confirm that Cognitive Project Management was taking a position on the interpretation of Restriction Delay values (for Start and Finish Restrictions.

    That there are two different possible ways to interpret such values (Work Performance and Passage of Time), and there is no consensus on the Scheduling World … is precisely why Cognitive felt the need to take a formal position on the matter.

    And as explained in both the blog article and the book (CPM Mechanics), while either interpretation can be assigned to these values, Cognitive felt it would be better for there to be only one meaning: Work Performance.

    In conjunction with this mandate, it points out that by precluding the Passage of Time usage, it forces a better practice, of using an Activity (rather than a lead/lag value) to depict (and, thus, explain) a gap of time.

    As for your example, Stipulation #1 still works, as a ZERO value simply means that no amount of Work Performance (by the Restricting Activity) is required as a prerequisite for the Restricted Activity to commence.

  4. Dave Black says:

    Another interesting blog subject about what lead / lag values actually mean and do the Restriction Delay Stipulations proposed by Cognitive Project Management make any sense. As evidenced by the preponderance of comments on some of the various scheduling blogs the topic of leads and lags is one of many issues that can become contentious quickly. At times some of the blog comments appear vey argumentative and I am sure if some of the participants were physically present they might even come to blows.

    From a Cognitive Project Management perspective the interpretation of lead / lag values does make sense and offer an alternative to the Dominant Project Management approach. However, whether the project management community in general will change the way they presently handle leads and lags is as the old TV show used to ask; the sixty fur thousand dollar question.

    The two possible lead and lag values of Work Performance or Passage of Time in my opinion are open to interpretation regarding Stipulation #1. I can agree with the interpretation of stipulation # 2 and #3. However depending on what is actually happening with the #1 stipulation I might interpret it differently.

    For example if I had a Start Restriction or a Finish Restriction without any value is that a work performance or passage of time. You could argue the same as the Default Restriction that it’s a passage of time, where x is a zero value. If I do have either a Start or Finish Restriction value it could possibly be a resource issue that may occur because nothing can be done until the appropriate resources become available. Agreed that a lack of resources would impact work performance yet you could still argue that a passage of time is required before work can actually begin.

    Either way its an interesting approach, certainly worth considering.

  5. Rafael Davila says:

    Do you know Primavera Risk cannot model risk on lag, that they recommend using an activity to represent lag risk tis in addition to using a different CPM engine that yields different resource leveled jobs. Guess everyone knows lag can be represented by an activity with the nuisance to require updates for passage of time.
    Even when any lead or lag can be substituted with an activity it does not make much sense to prohibit their use. Of course such response can teach the specifier about how ridiculous such a requirement is. It shall be enough to require an explanation for all lead and lags as a comment on the links table as provided by good software. This will make it transparent and will make it unnecessary for additional activities foreign to the way most contractors plan and manage their jobs.
    FS only will not prevent out-of-sequence, they happen under FS links as well.
    If you have to drive 100 piles and 100 pile caps, how would you schedule it?
    1. An activity pile per pile and an activity per pile cap? – 200 activities.
    2. Every 10 piles or every 20? – A sequence that will be broken so often and probably will never happen?
    3. Or overlapped group of activities as the contractor and project superintendent will plan it?
    Why those who know less usually insist on teaching the contractor how to plan their own jobs?
    No wonder the CPM schedule ends up as toilet tissue on the jobsite portolets.

  6. Rafael Davila says:

    If you are interested into exploring volume lag, real float values under resource leveling,
    selection of multiple resource leveling algorithms, statistical methods such as Monte Carlo and 3 scenarios approach (different to wrong original PERT that did never considered the possibility of changing critical path), an multiple WBS dictionaries among others you can download a demo at the following link;
    http://www.spiderproject.ru/enter.php?ver=prof&lang=eng
    Much of the discussion on this blog is considered under the unique functionalities of Spider Project and perhaps is also available on other software options many do not explore. Perhaps victims of one of the postulates on this Blog that warns about one solution that fits all.
    I believe the one with ultimate control of the means and methods shall not be limited by pre-conceived standards that are limited and at times in error. He shall be allowed to use the software of his choice on a lump sum contract, on other types of contract it might makes sense to specify a common software. Guess here again there is no such thing as a single solution that fits all.
    I believe the Government Agencies (federal/state/municipal) shall not be dictating how the contractor is to manage his work. They might ask for a specific WBS, they might ask for schedule submissions and updates as they have their own scheduling and portfolio needs, but not controlling the management tools.
    At home very frequently the Agencies specify software different to the contractor, yes brand specifying when it is considered wrong on government contracting, and they know it well. So wrong that for every occasion they shall ask for a waiver they never ask. This is government in the business of creating monopolies with their software friends, perhaps because of kickbacks, goes against the principles of free enterprise and perhaps against international agreements.
    The effect of this practice has been that every time a Government Agency specifies different software the contractor is forced to hire external scheduler to meet this requirement, outsourcing what shall never be outsourced. To me like outsourcing CIA functions to North Korea (yes that bad software, usually notorious for thousands Bugs, incapable of yielding resource leveled float and difficult to install). Under this scenario many government contractors will be forced to abandon integration of planning with their accounting, job costing, resource planning software, it no longer makes sense.
    I have seen so many horrors in the PMI as well as on the AACE International protocols that I believe referencing or mandating them is in abominable error.

  7. Rafael Davila says:

    Some say “performance of work,” while others say “passage of time.”

    I say both shall be used as appropriate. There are software that implement time lag as well as volume of work lag. Time lag is defined as passage of time while volume of work lag is defined as some volume of work. Volume of work lag duration is computed on productivity values and if a successor starts prior to the keeping of this value an out-of-sequence flag will be issued, similar to time lag but always active until all predecessor volume of work is performed. Why not having the best of both worlds and implement the functionalities correctly?

  8. Michael Neal says:

    [C] It is amazing to review schedules and see the amount of leads/lags negative/positive that are used. This I believe is a direct lack of proper planning and not knowing how to build a schedule. The government contracts do not allow for lead/lags. This makes for a schedule to have more activities. Lets say you have Level A 10,000 sq ft. The electrician is not going to wait till all of the walls are erected before starting his first rough in, that would not be value engineering. Since leads/lags cannot be used the area is broken into different compartments till an acceptable approach is approved. This will allow the schedule to be better maintained and updated. The terminology using restrictions provides a better understanding than just a SS, FF.

  9. Connie Bremer says:

    This blog does instigate something to think about. I have to admit that I hadn’t given much thought to these concepts. I never really asked, I just always took lead to indicate a predecessor has a specified lead in front of a successor, and lag to indicate it lags behind a predecessor by whatever value is assigned. I just always associated lead with predecessor and lag with successor. I’m not sure yet why we need both terms, it just seems to add unnecessary ambiguity.
    However, your example of the friends meeting for a drink brings a better understanding for me why the start and finish restrictions should only involve work performance and default restrictions involve time.

What is your opinion? We'd like to know!