A Precedence Diagramming Method (PDM) or Activity on Node (AON) diagramming method is a graphical representation technique. It shows the interdependencies of activities and is used in schedule development.
We use this method to draw the project schedule network diagrams, such as critical path network and chain network diagram.
Precedence Diagramming Method (PDM) or Activity on Node (AON) Method
The Precedence Diagram (AON diagram) shows activity relationships and is an important communication tool for stakeholders.
The precedence diagram is made of rectangles known as nodes. These boxes show the project activities. An arrow connects two boxes and shows the relationship. Therefore, these diagrams are also known as activity on node (AON) diagrams.
Type of Dependencies in Precedence Diagram
The PDM uses four dependencies:
- Mandatory Dependency
- Discretionary Dependency
- External Dependency
- Internal Dependency
Mandatory Dependency
This dependency is also known as hard logic, meaning it’s a fixed reality you cannot avoid. Starting the next activity depends on it.
For example, you cannot install the ceiling until you build all the walls.
Discretionary Dependency
This dependency, also known as preferential or soft logic, plays a role in optimizing resources.
For example, you can construct the four walls in any sequence. However, if constructing them in a certain sequence is beneficial, you build them in that order.
With discretionary dependency, you can change the sequence of activities as per your preferred logic.
External Dependency
The project management team has no control over an external dependency.
For example, you may need government approval before starting the next activity.
Internal Dependency
These dependencies are within the control of your project or organization.
For example, you cannot get a resource until it is free from another project.
Dependency Relationship in Precedence Diagram
The precedence diagramming method (PDM) uses four relationships:
- Finish to Start (FS)
- Finish to Finish (FF)
- Start to Start (SS)
- Start to Finish (SF)
Finish to Start (FS)
In this relationship, the next activity cannot start until the first is complete. This is the most common relationship in the AON diagram.
For example, to paint a wall, you first have to construct it. With finish to start, the first activity is building the wall, and the second activity is painting. You cannot start painting the wall until the wall is ready.
Finish to Finish (FF)
In this relationship, you cannot complete the next activity until the first is finished. Put simply, both activities should be finished simultaneously.
For example, let us say you are coding a program for a client. The client gives you the features after completing a milestone. You cannot finish coding until you get the client’s complete requirements.
Here, both activities should be finished simultaneously.
Start to Start (SS)
In this relationship, the next activity cannot be started until the first starts. Both activities should start simultaneously.
Example: To apply a coating on a wall, the wall must be cleaned.
Therefore, one team will clean the wall, and the second team will coat it. Both activities can start simultaneously.
Start to Finish (SF)
In this relationship, you cannot finish the next activity until the first starts.
For example, suppose you are moving into a new home, and your old home has to be demolished. In this case, you cannot move into your new home until it is ready. Hence, the second activity (construction of the new home) must be finished before the first activity starts (moving into a new home).
Put simply, when moving into your new home, you cannot start vacating your old home until the new house is ready.
Although this relationship is rare in an AON network, you must understand all the dependencies. It will help you draw the network diagram and develop the project schedule.
You may hear the term activity on arrow (AOA). This is a less commonly used technique in diagramming methods. The AOA method is a special case of the precedence diagramming method.
AOA diagram only uses the Finish to Start relationship. It shows the duration on the arrows; therefore, many experts call this diagramming method the activity on the arrow diagram. PERT is an example of this technique.
There is a difference between the AON and AOA diagrams. The AOA diagram emphasizes milestones (events), and the AON diagram emphasizes tasks.
How to Draw a Precedence Diagram
To draw a precedence diagram, you will analyze your work and create a Work Breakdown Structure down to the activity level.
Then you will create a table, list all activities, and sequence them.
The next step is to add relationships to each activity. You will add what activity comes next.
Finally, you will draw the diagram.
Benefits of Precedence Diagramming Method
PDM offers many benefits to project management:
- It helps you visualize relationships and dependencies among activities. This helps you plan and avoid risks. If any task is missing, you can easily identify it.
- You can identify critical activities and focus on them. Any delay in critical activities will delay your schedule.
- A project schedule network diagram is a good communication tool. Stakeholders can visualize activities and understand the schedule.
- Without the Precedence Diagram, you cannot develop your project schedule.
Summary
The precedence diagram method has an important role in project management. Your project schedule depends on it and is a good communication tool. It is commonly referred to as AON, where nodes represent activities. AOA diagram is a special case precedence diagram where nodes represent milestones, and duration is shown on the arrow.
This topic is important for the PMP exam. You may see one or two questions on this topic.
Here is where this post on Precedence Diagramming Method (PDM) or Activity on Node (AON) method ends.
Please share your experience using these diagramming techniques through the comments section.
I am Mohammad Fahad Usmani, B.E. PMP, PMI-RMP. I have been blogging on project management topics since 2011. To date, thousands of professionals have passed the PMP exam using my resources.
Pleaase can you help me sketch the network diagram for this job.?
ACTIVITY DEFINTION
A. Planning and design activity
Activity A will include preliminary plan and design of the Recreational Hall. In addition, the project manager during the duration of this task should confirm funding source and shortlist proposals submitted by constructors. This activity will take two (2) days.
B. Acquire subcontractor
The project manager and panel of selection members will conduct research and interview of the shortlisted contractors. Then confirm and acquire subcontractors. Activity B will take one (3) day.
C. Design package
Activity C is when project team finalize the building architecture, mechanical, electrical and plumbing infrastructure. In addition, design and do listing of cabinetry, appliances, and countertops, flooring, electrical, lighting and painting. This activity will take 5 days.
D. Review Scope of Work
DBTI project team and subcontractor to review the scope of work and design packages. This activity will take 2 days to complete.
E. Construction Materials Costs
Collect three quotation of all the required construction materials for costing and purchase. This activity should take 1 day.
F. City permits and inspect
The PNG Building board is responsible to approve the design as per legal standards and policies of PNG. This activity will take 2 days to complete.
G. Demolition
Demolition activity (G) will include creating of demolition plan; hire demolition equipment, secure waste container and truck for disposal of rubbish, hire demolition team and complete demolition. This activity will take 2 days to complete.
H. Demolition cleanup
Clean and prepare site for construction. This activity should take 1 day.
I. Procurement of materials
Purchase materials for the various construction. This activity will take 3 days.
J. Mechanical
Complete mechanical rough in, installation and testing. This activity will take 3 day to conclude.
K. Electrical and Computer infrastructure
Complete electrical and computer infrastructure rough in, installation and testing. This activity will take 10 days to complete.
L. Plumbing
Complete plumbing rough in, installation and testing. This activity will take 12 days to complete.
M. Paint
Paint the interior and exterior of the Recreation Hall. This activity will take 1 day to complete.
N. Cabinet/Countertops
Install cabinets and countertops. This activity will take 3 days to complete.
O. Installation of appliances
Installation of all kitchen appliances. This activity will take 4 days to complete.
P. Test and commission
Test refrigerators, dishwasher, microwave and over. If all appliances are working, commission for use. This activity will take 3 days to complete.
Q. Cleanup
Remove unused material and remove trash. This activity will take 1 day.
R. Turnover
Subcontractor sign off to DBTI. This activity will take 2 days.
?
b) ACTIVITY LIST and ATTRIBUTES
1. Activity A will kick off the project.
2. Once preliminary planning and design is completed, activity B can start.
3. Activity C can also start when A is completed.
4. Activity D is a successor of activity B
5. Activity C and D must be completed before activity E and activity F can start.
6. Demolition can start when PNG Building Board gives the approval of the construction.
7. After demolition is finished, clean up in preparation for the construction to start.
8. Purchasing of construction materials can start after receipt and approval of quotation.
9. After demolition cleanup and procurement of materials are completed, the project team can now start on activities J, K, L, M and N.
10. Installation of electrical appliances and equipment can start after J, K, L, M and N are completed.
11. Testing and commission of the Recreational Hall after O is completed.
12. Clean up after testing and commissioning of the refurbished building.
13. Project team successfully handover the property to the institution.
Draw a PDM network for a project with the following activities. Show all steps including removing redundant relations; and sequence steps.
– Activity B depends on A;
– Activity G follows E, F & D;
– Activity E depends on B and A;
– Activity F can start when D & B are completed;
– Activity C is followed by F and follows A;
– Activity D is dependent upon A and B.
Hello Shehan, you can find a few answers on the following page:
https://pmstudycircle.com/study-notes/
Consider a project that you have worked in. discuss the practicality of application, advantages and disadvantages of following theories.
1.Precedence diagrams
2.Resource aggregation
3.Resource leveling and smoothing
4.Crashing
5.Fast tracking
6.Line of balance
7.Linear programming
(please answer this questions as you can. for a research purpose)
Hi Fahad
Thanks for your post. This really helps. Can you also give an example of AOA diagram. This helps to clarify the difference between the two.
PERT is an example of aoa.
By looking at the dependency diagram, how do we know which is a start to finish, finish to start, finish to start and start to start? I understand the concept, but looking at the picture don’t know how to identify the difference?
I am referring to your book pmp question bank page 16
Pictures of dependencies are shown in this blog post. What you did not understand?
The example Mona as given in the PMBOK 5th edition makes the understanding clear. But the job of a security guard in shift does not fit into the definition of “Project”. However, to understand the Start-to-Finish concept, this example is fine.
I am in the midst of preparing an assignment and in one question I am asked to ” propose practical project management planning and execution guidelines for ensuring that projects are completed within reduced lead times, reduced cost while at the same tine still meeting specifications” where do i begin?
I came across a familiar example for Start to Finish activity.
Example: While farming in a field lets take two activities 1) Furrowing 2) placing seeds/saplings
Now apply the S-F principle here; The first activity is furrowing and the second activity is Placing seeds/saplings.. So the placing seeds cannot be finished until furrowing is started.(assumption made: placing seeds/saplings in a discrete manner)
if there was no relation on the arrow, how should calculate the successor? do we have to assume it or what?
There will always a relation between two tasks if they are connected by an arrow.
i may be wrong but FF seems to have a wrong example.
“For example, let us say that you coding a program for a client, and the client is providing you the characteristics of the program. In this case, you cannot finish coding for your program until the client gives you his complete requirements. Here, both activities should finish simultaneously.”
according to PMBOK: WRITING (PRE) a document to be completed to finish the EDITING (SUC). this is a better example.
in your example the actually requirements is the predessor and coding is successor. then how programming will finish when the client finishes the requirements? in real world you get a requirements list, then you develop and then finish. here i understand the requirements flowing and the programmer is coding as and when he gets a piece of requirement! :-)
good day
please assist ,
what is mean by tabulating various types of time dependencies that one may encounter when using the precedence diagraming method
Please read following blog post:
https://pmstudycircle.com/2012/07/precedence-diagramming-method-activity-on-node-method-scheduling/
Thank you for your respong…
I have read it and understand the blog post….. what i dont understand about the question is that how do you start to draw a table of those Dependencies….. what is on which column…..
Once again thank you…
Soon I will update this blog to accommodate your concern.
Thank you Fahad
You are welcome Confidence.
Guys don’t get panic….. its simple….. schdeuling techniques can be as follows:
1. bar chart / gantt chart
2. milestone chart
3. line of balance and
4. network diagram
i. critical path method (CPM)
note: network diagram for CPM could be either Activity on Arrow network (AOA) or Activity on Node (AON) / (Precedence Diagramming Method (PDM)). SO IF NETWORK IS DRAWN AND IT IS NAMED AS NETWORK THAN THAT IS TRUE …SIMPLY OBSERVE WHETHER IT IS AOA OR PDM….
ii. programme evaluation and review technique (PERT)
Well said Subash.
Can we find Float of an activity by the help of LS and LF values as we did in following by ES and EF values
ES of next activity — EF of current activity –1
like
LS of next activity — LF of current activity –1 (is this formula is OK)
Thanks
Read below given blog post:
https://pmstudycircle.com/2013/03/total-float-versus-free-float/
What’s the exact difference between precedence diagram and a network diagram?
Precedence diagrams are network diagram.
There’s a question which shows a diagram like yours in this blog and then puts following question:
51. The above diagram is called:
a. Activity on arrow network diagram
b. Network diagram
c. Precedence diagram
d. Gantt chart
If they are the same, then why the answer is “Precedence diagram”? I chose B, which is a wrong answer.
Can you please help me with that?
Let me explain it to you with an example.
You have been shown a pineapple and asked to identified it. The options are:
a) Pineapple
b) Orange
c) Fruit
d) Banana
Now let’s say you have selected answer c that is “Fruit” .
Is this a wrong answer? No, it is not. It is a fruit. This is a correct answer but not a the best answer given in the options.
You have been asked to select the best correct answer; which is pineapple.
So the correct answer would be: pineapple.
Likewise, in your question, you have to chose the best answer.
Hope it helps.
That’s exactly the kind of question that I hate. It doesn’t test your ability as project manager. If you check PMBOK 5th edition, figure 6-11, it shows a precedence diagram and the figure title is network diagram! How can we not make this mistake like Lee did?
Hi Fahad,
i always get confuse in dependencies related question. Any tip to clear the concept or to read and understand question in a way so that i can answer.
You can refer my blog post on critical path method.
https://pmstudycircle.com/2014/01/critical-path-method-cpm-in-project-management/
Please i want full diagram for two storey building. Thanks
Sorry Joseph, I don’t have diagram for any two story building.
This is very helpful to me. Btw, do you have any example or case study or maybe real data involving PDM network? I’m doing a research on this topic and I found really hard to obtain project schedule with PDM networks. I really appreciate it if you could give me some suggestions. Thanks.
Sorry Atiqah, I don’t have any case study on it.
Hi,
Could you tell me how to calculate the forward and backward pass for a network, maybe using an example exercise. Just to be clear how the steps go. I feel it will give me a more clear picture.
Thankyou
I mean the forward and backward pass for a PDM network.
Here, everything is explained with an example:
https://pmstudycircle.com/2014/01/critical-path-method-cpm-in-project-management/
An example for SF could be turning ON the room heater in a hotel room (Activity 1) and ensure ‘comfortable & satisfied’ check-in of the guests to the room by the front office assistant/bell man (Activity 2), the ‘comfortable & satisfying’ check-in of the guests to the room by the front office assistant/bell man can be considered COMPLETE only when the room heater has been turned ON and the room has STARTED to warm up.
I agree SF is a very rare scenario in a PM context.
Well said Kay.
I am in the midst of preparing an assignment and in one question I am asked to conduct a forward and backward pass using the “precedence diagram method (Activity on node – AON)” and the next questions asks me to display the completion time for the entire project in a network diagram.
My question is, wouldn’t I just refer my professor to the network diagram in the previous question? the forward and backward pass essentially calculates my project duration. How are these diagrams I am being asked for different from each other?
Kristen,
This free pdf download is an excerpt from the Head First PMP book … it might help you. I’ve been putting off this section, deciding Earned Value was easier, lol, so not going to be able to offer much help.
http://www.headfirstlabs.com/PMP/criticalpath/HeadFirstPMP_CriticalPathDrill.pdf
~Z
Forward pass helps you to identify the early start and early finish of an activity, while backward pass helps you to find late start and late finish of an activity.
please data provide and solve as reference.
This is very helpful site Thanks.
Can some one post lots of CPM (Time management) and costing examples? I started solving practice test, I found that, I’m spending lot of time thinking about complicated CPM and costing problems. I need to do more practice with this.
I will appreciate, if some one suggest me good web sites for these examples.
Thank you.
Hello Mahesh,
Not specifically for time and cost management questions, but you can find some sample PMP exam questions here at
https://pmstudycircle.com/pmp-questions/
Really helped me in my assignment….fanks
I am glad that it helped you.
I don't agree with the explanation above for START-FIHNISH dependency. When you say
"…you can not move to your new home until it is ready. Hence, second activity (construction of new home) must be finished before first activity starts (you start moving into new home)…"
you're just defining a FINISH-STAR dependency where the precedent activity is CONSTRUCTION OF A NEW HOME and the subsecuent activity is MOVE IN NEW HOME.
Think in this way –
You have to vacate your home as first activity, and moving into new home as second activity.
Now, you can not move into new home until it is completed.
This example is just to make picture clear to you. Anyway, if you can suggest any other good example, you are welcome…
And, thanks for stopping by!
I agree with Spanish. I am struggling with an example for start to finish that makes sense. Your example appears to simply be poor sequencing of activities. I’m hoping there is a sr pm out there who can help.
Hello Newb,
Start to finish is very rare sequence of activity and not used very frequently. While writing this blog post, I tried to find a good and simple example of it. Although, I found few examples of this activity but they were all very typical and not easy to understand.
Therefore, I thought myself and came up a simple example of this sequence. I agree with you that this may not be the perfect example, but if you consider the scenario given in the example, I believe that it serves its purpose.
I found a good example of Start-to-Finish in PMBOK, 5th edition:
First security guard shift (successor) cannot finish, until the second security guard shift (predecessor) starts.
You may think that this is just a matter of re-ordering your activities and describes a Finish-to-Start relationship. But, in this case, you really cannot Finish the first shift, until the second shift has started. Hence, describing a Start-to-Finish relationship.
Atleast , made sense to me :)
Thanks Mona for sharing your understanding.
ASSALAM O ALAIKUM SIR I am student of final year civil engineering dear Sir I WANT TO ASK ONE THING Regarding PDM( PREDECCENCE DIAGRAMING METHOD) . SIR IN PDM METHOD DOES AN ACTIVITY OR TASK STARTS FROM SAME DAY OR NEXT DAY ?
SIR EARLIER WE R TOUGHT AS AN ACTIVITY STARTS FROM NEXT DAY AND WE DO +1 IN THE DEPENDENDENCIES IF ZERO ACTIVITY THEN WE DO NOT ADD +1 IN ITS DEPENDENCY IN PDM METHOD IS IT CORRECT ANSWER IF NOT THEN PLZ GIVE ME A SUITABLE & CORRECT ANSWER TO CLEAR MY CONCEPT THANKS REGARDS
Fahad Hussain
If you starts from zero, no need to add 1.
but in this case, first security guard is the predecessor if you see the sequence of activities, so the first activity cannot be finished until the second activity starts. But this is the contradictory to the SF definition
example is good but alter the words predecessor and successor
……
What did you mean?
Makes sense, Thanks
Perhaps I have a very local example. In the night time in your room, there is a high power bulb is en-lighting. you have to get a low power bulb enlightened. So, you go to the switchboard and first switch on the low power bulb then you switch off the high power bulb.
hi, i am just a student and i may not be right but i wanted to quote an example of start to finish, here..we have an automatic power (electricity) generator at home… as soon as the power goes off it starts automatically after 5 seconds..
but the again when the power(electricity) is resumed.. it waits for 5 secs and automatically shuts off.. so u can say it waited for the electricity to start and then finish its working.. it cant finish working unless the electricity is started.
LET US HAVE A EX,
IF SHIFT 2 WATCHMAN HAS TO ASSUME THE CHARGE, SHIFT ONE WATCHMAN HAS TO HANDOVER IT TO FIRST ONE. FINISH THE RESPONSIBILITY/CHARGE TO STARTOR ACQUIRE THE NEW ONE
The difference between finish-start and start-finish is that start-finish cannot finish before the next task starts. In finish-start you can start next task whenever that previous task is done, this is how we schedule most things. In start-finish the finishing of the first task is dependent on the next task picking it up. These are likely to be time-sensitive.
The shift work example is one of the best. Current shift cannot leave until next shift comes online. If next shift does not arrive, current shift must stay on. Your task is finished ONLY when the next shift starts, you cannot finish your task otherwise. Finish is dependent entirely on the start of the next task (shift).
Another simple example would be holding a heavy load above your head. You cannot finish until the next person comes in to transfer the load, otherwise you’ll drop it on yourself and die. This isn’t a task that can be finished by person A and resumed later by person B; person B must start the task in order to relieve A. Person A cannot decide he is finished until B starts.
Not sure there are other contexts where this is useful outside of relief crews, but if you manage a 24hr operation it becomes quite important.
Thanks Loyang for sharing examples.
I need some formules fore solving this kinds of network.
Hope this blog post will help you:
https://pmstudycircle.com/2014/01/critical-path-method-cpm-in-project-management/
S-F are used when the predecessor activity is a start milestone and the successor a finish milestone, because milestones either have a start or a finish.
They can also be used in cases where the successor activity is in progress prior to completion of the predecessor, but can’t be completed until the predecessor is started. This functions sort of like a F-S with negative lag, but can more accurately reflect the logic between two activities. The predecessor doesn’t drive the start of the successor, something else has to.
I presume that the author is some what correct in giving the example, as he says that the demolition of the older house (S) can’t be attained until there is a complete transfer to the new constructed one(F). Thanking you…
Yes Mayeena.