PMGT1865 Chap.6 Overlapping Relationships and Link Lag
Overlapping Relationships and Link Lag
Everything up to Week 3 assumed a finish to start world in which nothing may begin until the activity in front of it has finished entirely. That assumption was made to keep the critical path method simple, and real projects break it constantly.
An overlapping network diagram shows some activities as partially or fully concurrent, and drawing one both models the work more realistically and shortens the schedule, because a successor no longer waits for a whole predecessor. It also lets resources be deployed earlier, which smooths the demand curve, and it makes concurrency visible on the diagram where it can be managed rather than discovered.
Four relationships are possible between two activities, since each has two ends, and the most common compound link pairs two of them. The chapter also separates two things that share the word lag: the lead or lag time a planner attaches to a link before any pass, and the link lag computed from the dates afterwards.
What this chapter covers
- 01
Why overlapping is drawn: shorter schedules, earlier resource deployment, visible concurrency
- 02
Overlap as a claim about the work, and the physical reason that has to sit beside every one
- 03
Finish to start, start to start, finish to finish and start to finish, and what each constrains
- 04
The compound link, and the pairing this unit takes as its default
- 05
Link lag as a measured gap after the forward pass, and the formula that defines it
- 06
Lead or lag time as a planner's input, and why the two are confused on simple chains
- 07
The four forward pass rules, and reading each one off the name of its link
- 08
Why two of the rules produce a finish and therefore need one more subtraction
- 09
Taking the maximum where an activity carries several incoming links
- 10
Split activities against the simplified precedence network, and when each drawing is useful
A forward pass through mixed link types
- +1Demolition has no predecessor, so it starts at 0 and finishes at 5.
- +1Plumbing follows by finish to start with no lead, so its early start equals demolition's early finish. It runs 5 to 11.
- +1Tiling follows by start to start with a lead of 3, so the rule works from the predecessor's start: 5 plus 3 gives an early start of 8, and tiling runs 8 to 16, overlapping plumbing for three days.
- +1Joinery follows by finish to finish with a lag of 2, so the rule gives a finish: 16 plus 2 is 18. Subtract the four day duration to place the start at 14, so joinery begins while tiling still has two days to run.
- +1Final inspection follows by finish to start, so it runs 18 to 19. The project duration is 19 working days.
- +1Check the overlaps are real. Tiling starting three days into plumbing assumes three days of plumbing exposes enough tileable surface, and joinery starting two days before tiling ends assumes the joinery run does not sit on the last tiles laid. Both are claims about the site and both belong on the diagram.
Key terms
- Overlapping network diagram
- A network in which some activities are shown as partially or fully concurrent. Drawing one removes the assumption that work can only be scheduled sequentially, and it usually shortens the overall duration.
- Start to start link
- A relationship in which the following activity cannot start until the preceding one has started, usually once a portion of it is complete. The required pre-completed portion is what the lead time measures.
- Finish to finish link
- A relationship in which the following activity cannot be completed until the preceding one has been fully completed. Both activities can start independently of each other.
- Start to finish link
- The least common of the four, in which the start of the preceding activity defines the finish of the successor. Switching off an old system once a new one goes live is the standard illustration.
- Compound link
- Two links acting on the same pair of activities, conventionally a start to start together with a finish to finish. Each carries its own offset, and both constraints must hold simultaneously.
- Lead or lag time
- The planned offset a planner attaches to a link before any pass, representing a deliberate delay or a physical wait such as curing or an approval turnaround.
Overlapping Relationships and Link Lag FAQ
What is the difference between lead time and link lag?
One is an input and one is an output. Lead or lag time is the offset a planner writes on the link before any pass, either because the work forces a wait or because the successor is being deliberately delayed. Link lag is computed afterwards from the calculated dates, as the successor's early start minus the predecessor's early finish. They can carry the same value on a simple chain, which is exactly why they get confused.
Label the diagram so a reader can tell which is which.
When is an overlap legitimate?
When there is a physical or contractual reason the successor can begin before its predecessor is complete, and you can state it. Starting tiling three days after plumbing starts is legitimate only if three days of plumbing produces enough finished surface to tile.
Overlap invented to make a schedule fit is the standard way an overlapped network becomes undeliverable, and unlike a logic error it produces no arithmetic symptom at all, so writing the reason beside each overlap is the only defence.
Why do two of the four rules feel harder than the others?
Because finish to finish and start to finish compute a finish for the successor rather than a start. To place the bar you then subtract the duration, and that extra step is where errors creep in.
It is also why a finish to finish link can produce a successor that appears to start earlier than expected: that is not a mistake, it is the link doing its job, and it is the reason a compound link can hand you an implied duration you did not choose.
Assessment move
Learn the four rules as one rule read twice rather than as four things to memorise. The first word of the link name tells you which end of the predecessor you start from, and the second word tells you which end of the successor you land on. Finish to finish starts from a finish and lands on a finish.
Once that clicks, the only remaining work is remembering to subtract the duration after a finish rule and to take the maximum wherever an activity has several incoming links. Practise on tables with mixed relationships rather than on single links, because the maximum is where mixed tables get interesting: an activity with a start to start link and a finish to start link gives two candidate early starts and the later one binds.
Finally, get into the habit of computing the pure finish to start duration alongside the overlapped one. The difference is what the overlap bought, and being able to state it turns a calculation into an argument.
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