Structure Diagrams and Flowcharts

A structure diagram records how a problem is divided into parts and how those parts link together. Lines join a subsection to the part it belongs to, so the diagram shows the structure of the problem. It does not show the order of every step in a process, or any decisions made along the way.

A flowchart does a different job. It shows the steps of a process in the order they happen, including decisions and loops. For example, a flowchart for checking library books might scan a book, compare its due date with today's date, branch Yes or No depending on whether it is overdue, then loop back to scan the next book. Decisions and loops are both features of a flowchart.

This is why a structure diagram has no decision shapes, no Yes/No branches and no loop-back arrows. A decision shape that asks a question and splits into two paths belongs to a flowchart, as does an arrow returning to an earlier step. Adding either to a structure diagram mixes up two different tools: one shows the parts of a problem, the other shows the order of a process.

If you were planning a school science fair, the structure diagram could show Build stands and Label exhibits under Set up venue; those lines tell you where each task belongs. It would not show a decision about whether it is raining or a loop for greeting each visitor in turn. Those are details of a process, so they belong in a flowchart.