In section 220.127.116.11 of version 6 of the guide, (Precedence Diagramming Method) it states that "closed loops are also not recommended in logical relationships." Does anyone have a practical example of this?
Close the book when completed reading = Complete reading when closing the book
I would look at the definition of closed loops. The "closing" path of a diagram is often the feedback returned into the initial step for adjustment or modification.
For example, if a car is set to cruise control.
Step 1: Controller sends command to increase throttle
Step 2: Controller measures current speed of car.
- If current speed of car = target speed of car, restart Step 2
- If current speed of car is more than target speed, reduce throttle and return to Step 2
- If current speed of car is less than target speed, return to Step 1
This set of steps is ideal in an operations scenario, or a going-concern scenario. Something which needs constant correction and readjustments. Project tasks, I would say, as specific to achieving the project objectives.
Any project feedback mechanisms (issue review, risk review, defects correction, etc.) are usually sequential (e.g., after identifying defects, we would not revert back in time to the earlier build task - but we would have a new build task to make necessary corrections).
The two quoted relationships in the original post are not analogous. The former states that I will close the book only when I complete reading. It is a FS relationship between [Complete Reading] and [Close Book] tasks.
The latter, I'd say, is pretty much how lazy students behave; "I have read two chapters and have closed the book. Therefore I have completed my reading on this subject". It does not follow. Saving Changes...
"Closed loops" in PDM networks are accidents - typically called "circular logic" in the software. E.g. A-B-C-D-A; or in your example, "complete reading" - "close book" - "complete reading." To say closed loops are "not recommended" is an understatement, as their existence prevents completion of the schedule calculations. Closed loops are made more likely by certain scheduling habits, like non-unique activity names or logic on summary tasks, that are discouraged. Practical examples: none.
Real projects do include some feedback loops, especially with respect to inspection and testing, but these cannot be modeled directly using PDM. Saving Changes...