The precedence diagrams used for critical path analysis are normally constructed by connecting activities with dependencies (links) that illustrate how one activity must be preceded by another. These are sometimes known as ‘mandatory dependencies’.
For example, building a wall must be preceded by laying a foundation:
To put it another way, it is mandatory that the foundation be laid before the wall is built.
However, what if you were building two walls – one at the front of the house and one at the back? These walls are independent in purely technical terms. It doesn’t matter what order you build them in and they could, in fact, be built at the same time.
It may be that you make a decision that they should be built in a particular order. Perhaps the customer wants the one at the front done first or you want to see how the simpler brickwork at the back of the house turns out before you try the more complicated design at the front.
Either way you can choose to enter a dependency between the two activities. This is known as a discretionary dependency.
Planners are often tempted to use discretionary dependencies to show the flow of resources, e.g. a dependency between the back wall and the front wall is used because we only have enough bricklayers to work on one wall at a time.
This may be a valid way of resource limited scheduling if you want to do that manually but, if you are using planning software to perform resource scheduling, bear in mind that this kind of discretionary dependency greatly reduces the options the software has to find the optimum schedule for use of limited resources.
Be sure to document your discretionary dependencies. It is inevitable that you will need to review the network diagram to respond to issues that arise and it should be easy for everyone involved to differentiate between dependencies that must be retained and those that can be changed.