The key to successfully levelling is to make the diagrams balance. For example, if a second level diagram expands a first level process then all the inputs to the process must be inputs to the second level diagram, and all the outputs from the process must be outputs on the second level diagram. Moreover, there must be no other inputs and outputs. To be particular, all the inputs and outputs of the system which appear on the context diagram must appear on the level one diagram and there should be no other inputs and outputs on the level one diagram.

This does not mean there can be no changes to the higher levels of a set of diagrams. When producing a lower level diagram, the software engineer may realise that a new input is needed for the process to be able to carry out its task. In which case, the software engineer should add this data-flow as an input and then add the input as a data-flow to the original process. If needs be, this input may be added at several levels higher up. The software engineer may add new outputs in the same way.

As long the diagrams always balance, inputs and outputs can be added and removed wherever necessary.