Often enough in our classes we find professionals confuse “lead” with “fast track”, both scheduling terms. In the last few days, there are pictures posted on Facebook that is showing this confusion.
You can access the picture via this link. Via this same link, one can see the original post and various responses.
In scheduling, as the project management team is developing the project schedule, there are various techniques to use to determine the logical relationship between activities. Among those techniques are lead and lag, schedule compression, dependencies determinations, among others. Schedule compression includes crashing and fast tracking.
The main confusion is between fast tracking and lead since they both involve overlapping of activities or phases.
Lead and Lag
Assuming most practitioners will start scheduling by showing all activities as Finish to Start (FS); per the next figure. From this figure, the total duration of these two activities is 20 days since they are back to back.
The next step will be for the scheduler/planner to determine if the FS relation is a must. In other words, does ‘B’ has to wait for ‘A’ to finish before it can start? If yes, we maintain the FS relation. However, if no we can consider overlapping.
Let us say ‘B’ can start 4 days before ‘A’ finishes; there is no need to wait. In this case, we can overlap them by 4 days as the next image shows.
In this case, because of the overlap, the total duration is 16 days. We show this as ‘FS – 4’.
In this previous point, we started with ‘finish to start’ relation and then overlapped by 4 days. In other words, the reference point was the end of ‘A’.
Lag – 1
Let us look at this differently. Let us assume that the relation was Start to Start (SS). This means ‘A’ and ‘B’ can (or should) start together. However, the team determined that it is better to wait 4 days after we start ‘A’, to start ‘B’. In this case, we have what we call an SS relation with a lag of 4 days (SS + 4). The lag is 4 days from the start of ‘A’. In this case, we use the beginning of ‘A’ as the reference (in the previous case we used the end of ‘A’ as the reference). Graphically, the bar chart is identical, but the network diagram is slightly different (due to the change in relation).
In both of the above cases, the overlapping is LOGICAL. Meaning, NO RISK (more on this later) in overlapping.
Lag – 2
Let us go back to a finish-to-start relation.
In this case, let us say that once ‘A’ finishes, we have to wait a certain amount of time before ‘B’ can start. This is an FS relation with a lag, and we show it as FS+X; X is the waiting time. The next image shows this case.
Example of the above
The picture that triggered this article is a good example. We are assuming that, in the picture, the guy on the right is painting while the guy on the left is doing touch up or fixing things. In real life, we can start by saying painting-fixing is a finish to start relation but does it have to be? If one person is doing the work maybe. However, if we have enough workspace for two people (or two crews) to work then while the first one is painting, the second one could be fixing (touch up, etc.) but it is not total parallel. In this case, we can use FS with a lead or SS with a lag – will give us the same result.
Graphically, fast tracking looks exactly the same on a bar chart as an FS with lead or SS with a lag.
However, what some of the respondents to the Facebook post (most are saying the image represent fast tracking) is that in this technique the overlap is FORCED. What this means is that logically we should not overlap. However, due to schedule pressure and the need for schedule compression (which is not preferred during planning) the team may decide to force an overlap but by doing so they are taking risks.
An example of this, let us say activity ‘A’ is the design for a piece of equipment (pump, compressor, heat exchanger, high voltage transmission tower, large server …). Activity ‘B’ is the foundation design for this item. Logic says finish ‘A’ first, the design of the item, to determine its weight, forces, etc. Then use these design data from ‘A’ to complete ‘B’, the foundation design. Again – logic says Finish-to-Start.
However, to save time, one can say, “cannot we start the foundation design (B) based on preliminary data from A?” Yes, we can but one must recognize that the data is preliminary; meaning not checked or verified and as a result it could change. This is a RISK. Although this is a risk, management might accept it to save time. If the final data from ‘A’ matches the preliminary data – great, the risk did not occur. However, if the final data was different (the risk occurred), this might require modification to B, which means lost time and cost. In general, the added cost or time – would have been justified when the decision was made to overlap = fast track.
We hope this clarify the confusion.