The AACE Method
It is possible that AACE adopted this technique sometime after the software Primavera Project Planner (P3) was released in 1985. The technique describes the method that P3 uses to compute a PDM network. I learned this technique well before becoming involved with AACE and believed that it was the only way to correctly calculate the CPM. All of my scheduling associates also use this technique and were unaware of another technique.
Rule 1. Day numbers for a CPM network begin with Day 1.
Rule 2. For each activity without any unresolved predecessor relationships, you first compute the start and finish day numbers. Given a particular Early Start day number (ES) and the Activity Duration (Duration,) you compute the Early Finish day number (EF) using the following rule,
EF = ES + Duration – 1.
Rule 3. Once the ES and EF day number have been computed, you compute the ES for a finish-to-start (FS) relationship to the successor activity using the following formula,
ES(successor) = EF(predecessor) + Lag + 1.
The other three relationships are calculated as ‘normal’ without the “+ 1” adjustment.
Rule 4. Activities are star used as having begun in the morning before work begins and having ended in the evening after work has ended.
Rule 5. Once day numbers are computed, they are converted into dates using the work calendar for that activity without any further modification.
The Aggie Method
It is possible that this technique is older than the AACE Method. This appears to be the technique used by Microsoft Project to compute a PDM network. This PDM method is very similar to the method used in ADM calculations. The Aggie Method is a widely taught method and its practitioners also seem to be unaware that another technique exists.
Rule 1. Day numbers for a CPM network begin with Day 0.
Rule 2. For each activity without any unresolved predecessor relationships, you first compute the start and finish day numbers. Given a particular ES and the Duration, you compute its EF using the following rule
EF = ES + Duration.
Rule 3. Once the ES and EF day numbers have been computed, you compute the ES for a FS relationship to the successor activity using the following formula,
ES(successor) = EF(predecessor) + Lag.
The other three relationships are calculated using the same formula, only adjusting the start and end points of the relationship as required.
Rule 4. Activities are star used as having begun and end in the evening after work has ended
Rule 5. Since the day starts at the end of the day, you must adjust all start day numbers by first adding 1 work day and then converting them to dates using a work day calendar. Finish dates are converted directly without adjustment.
Conclusion
So which method is right for you? Well if you plan to take the upcoming AACEi Scheduling Certification test, you had better know and use the AACE Method. People using the Aggie Method will be marked incorrect and will probably not pass the certification exam. Those using P3 to compute their CPM should also use the AACE Method if they wish to understand how the underlying calculations made.
If you are a slightly-worried, Senior Lecturer at the Texas A&M University, College of Architecture, Department of Construction, and you have a very popular textbook using the Aggie Method, I would suggest that you keep on using the Aggie Method. Mr. Feigenbaum, I would like to take this opportunity to apologize for ruining your weekend in early August of this year.
Either way, I think that from now on, we all should first preference our discussion of how to compute a PDM CPM network by first saying which method we are employing.
Summary
It appears that for the past 15+ years, there have been two, formalized and competing methods for computing a PDM CPM. The really interesting thing about this fact is that apparently no one knew this. The chart below will summarize the two methods,
If we were to use both methods on the same five, 1-day activities in a serial chain of FS relationships, the following would summarize both calculations,
One may proceed down the left or the right side of the table above. Each method produces the same calendar dates as shown in the center column.