Precedence Diagramming Method (PDM): The 4 Types [2026]
A. Togay Koralturk, Best-Selling PMP Author
Last updated on September 17, 2026
8 min read
Ask a new project manager how two tasks connect and you will hear one answer: this one finishes, then that one starts. That covers most of a schedule, but not all of it — sometimes two activities start together, finish together, or overlap on purpose, and modeling those relationships wrong quietly warps the timeline. The precedence diagramming method is how professionals capture every relationship precisely. This guide covers the precedence diagramming method in full — what it is, the four dependency types, leads and lags, and how it shows up on the PMP and CAPM exams.
What is the precedence diagramming method?
The precedence diagramming method (PDM) is the standard technique for building a project network diagram, in which each activity is drawn as a node (a box) and arrows represent the logical relationships between them. Because the activities sit in the boxes, PDM is also called activity-on-node (AON). It is the method PMI teaches and the one virtually every scheduling tool — Microsoft Project, Primavera, and the rest — uses under the hood.
PDM matters because a real schedule is not a straight line of "finish this, start that." Some work overlaps, some work must end in sync, and some dependencies are hard physical constraints while others are just preferences. PDM captures all of that with a small, precise vocabulary of relationship types, leads, and lags — so the network you draw actually behaves the way the project will. Get the relationships right and the critical path and float fall out of the network correctly; get them wrong and the schedule lies to you.
The four dependency relationships
PDM connects any two activities with one of four logical relationships. The name always reads predecessor-to-successor — so "finish-to-start" means the predecessor's finish controls the successor's start.
| Relationship | Meaning | Everyday example |
|---|---|---|
| Finish-to-Start (FS) | The successor cannot start until the predecessor finishes. The default and most common. | You cannot start painting a wall until the wall is built. |
| Start-to-Start (SS) | The successor cannot start until the predecessor starts. | Pouring concrete can start once leveling the ground has started (they run in parallel). |
| Finish-to-Finish (FF) | The successor cannot finish until the predecessor finishes. | Editing cannot finish until writing finishes. |
| Start-to-Finish (SF) | The successor cannot finish until the predecessor starts. Rare. | The night shift cannot finish until the day shift starts (a handover). |
The vast majority of dependencies you will ever model are finish-to-start — it is the intuitive "do this, then that." Start-to-start and finish-to-finish let activities run partly in parallel, which is how schedules get compressed. Start-to-finish is genuinely rare and mostly shows up in shift-handover or just-in-time scenarios; on the exam, its main job is to test whether you truly understand the naming convention. Here is how the four look on a timeline:
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Leads and lags
Any of the four relationships can be fine-tuned with a lead or a lag — the two modifiers that shift a successor's timing without changing the logic:
- A lead pulls the successor earlier, overlapping it with the predecessor to accelerate the schedule. A lead is expressed as a negative value. Example: with a finish-to-start link and a two-day lead, the successor can start two days before the predecessor finishes.
- A lag pushes the successor later, inserting a deliberate wait. A lag is a positive value. Example: after pouring concrete (predecessor), you add a three-day lag before building on it, so it can cure.
Put dates on one link and the effect is obvious. Development finishes on day 10. A plain finish-to-start means testing starts day 10; add a 3-day lag (waiting on a staging environment) and it starts day 13; apply a 2-day lead instead and it starts day 8, overlapping the last two days of development. Same two activities, same relationship type — the modifier alone moved the start across five days, which is exactly why leads and lags are how schedules get tuned without changing the work.
Leads and lags are not padding or estimating error — they are real, intentional properties of the relationship, and they change the network's calculated dates. Applying a lead to overlap activities that used to be sequential is exactly what fast-tracking does to compress a schedule, which is where judgment (and exam questions) comes in.
PDM vs. the arrow diagramming method
PDM's older cousin is the arrow diagramming method (ADM), or activity-on-arrow (AOA). The difference is what the arrows carry:
- PDM / activity-on-node puts each activity in a box and uses arrows only to show dependencies. It supports all four relationship types plus leads and lags.
- ADM / activity-on-arrow puts each activity on an arrow and uses nodes for events. It supports only finish-to-start relationships and sometimes needs "dummy" activities (zero-duration arrows) to express certain dependencies.
Because PDM handles overlap, sync, and lead/lag natively and ADM does not, PDM won. Today, "network diagram" and "PDM" are used almost interchangeably, and ADM is a historical footnote you may still see referenced on the exam.
PDM on the PMP® and CAPM® Exams
On the PMP exam, PDM shows up two ways. The first is recognition: knowing that PDM equals activity-on-node, that there are four relationship types, and that a lead is negative while a lag is positive. The second, harder way is judgment — a scenario proposes overlapping two activities to save time, and you have to decide whether that is safe. The key discriminator is the type of dependency: fast-tracking (applying a lead to overlap sequential work) is appropriate for discretionary (preferential) dependencies, but not for mandatory (hard, physical) ones, where overlapping risks doing work on an incomplete input and reworking it.
The trap the PMP loves is a stakeholder pushing to overlap a mandatory dependency to hit a date. The professional move is to classify the dependency first, not to apply the lead reflexively. The CAPM tests the same material more directly — matching the four relationship types to definitions, converting leads and lags, and reading a small network — with less situational weight. Our PMP Complete Study Guide drills the four relationships and the fast-tracking rules until the distinctions are automatic.
PMP Practice Question: PDM Relationships
A schedule shows Prep (4 days, starting day 10) linked to Painting by a start-to-start with a 2-day lag (SS+2) — painting may begin once the first prepped rooms are ready, two days into the prep work. A scheduler tidying the file proposes re-entering the link as finish-to-start with a 2-day lead (FS−2), noting that both give Painting the same day-12 start and FS links are "cleaner to read."
What should the project manager do?
a) Approve the change, since with Prep at 4 days both links produce a day-12 start for Painting and are therefore mathematically identical.
b) Approve the change, since finish-to-start is the standard relationship and schedules should avoid start-to-start links where an FS equivalent exists.
c) Keep the SS+2 link, because the two forms only coincide at Prep's current duration — if Prep grows to 6 days, FS−2 moves Painting to day 14 while SS+2 keeps day 12 — and the link must encode the event that actually constrains Painting.
d) Keep the SS link but remove the 2-day lag, since lags conceal float and the wait should be modeled as an activity instead.
Correct answer: C.
Rationale: The equivalence is an accident of Prep's current duration, and seeing that requires running both links twice. Today: SS+2 starts Painting two days after Prep starts, day 12; FS−2 starts it two days before Prep finishes, also day 12. Now let Prep slip to 6 days: SS+2 still says day 12, because it is tied to Prep's start, but FS−2 now says day 14, because it rides Prep's finish. The two links behave differently under exactly the condition schedules exist to manage — change. A dependency's job is to encode which event constrains the successor, and here the constraint is real prep areas becoming ready early in the work, an event tied to Prep's start, so SS+2 is the truthful model and the "cleaner" rewrite quietly changes how the schedule reacts to slippage. Choice a) is correct arithmetic today and wrong logic tomorrow; choice b) invents a standard that does not exist, since all four relationships are equally legitimate PDM links; choice d) misstates lags — a lag is explicit, visible modeled waiting, not hidden float, and converting every wait into a dummy activity clutters the network without adding information. To face more questions where the right answer requires computing both futures, work through our PMP practice exams or, at the entry level, our CAPM practice exams.
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Frequently asked questions
What is the precedence diagramming method?
The precedence diagramming method (PDM) is the standard technique for building a project network diagram, where each activity is drawn as a node (a box) and arrows show the logical relationships between activities. Because activities sit in the boxes, PDM is also called activity-on-node (AON). It is the method PMI teaches and the one used by scheduling software.
What are the four types of dependencies in PDM?
PDM uses four logical relationships: finish-to-start (the successor starts after the predecessor finishes), start-to-start (the successor starts after the predecessor starts), finish-to-finish (the successor finishes after the predecessor finishes), and start-to-finish (the successor finishes after the predecessor starts). Finish-to-start is the most common; start-to-finish is rare.
What is the difference between a lead and a lag?
A lead pulls the successor activity earlier, overlapping it with the predecessor to accelerate the schedule, and is expressed as a negative value. A lag pushes the successor later, inserting a deliberate wait — such as curing time for concrete — and is expressed as a positive value. Both change the network's calculated dates.
What is the difference between PDM and the arrow diagramming method?
PDM (activity-on-node) puts each activity in a box and supports all four relationship types plus leads and lags. The arrow diagramming method (activity-on-arrow) puts activities on arrows, supports only finish-to-start relationships, and sometimes needs dummy activities. PDM is the modern standard; ADM is largely historical.
Is the precedence diagramming method on the PMP exam?
Yes. PDM is a core PMP schedule topic. The exam tests recognition of the four relationship types, the difference between leads and lags, and — more importantly — the judgment of when overlapping activities (fast-tracking) is appropriate, which is only for discretionary dependencies, not mandatory ones.
Is the precedence diagramming method on the CAPM exam?
Yes. The CAPM tests PDM within its predictive methodologies domain, typically by asking you to match the four relationship types to their definitions, apply leads and lags, or read a small network. The questions are more direct and carry less situational judgment than the PMP's.
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