How to divide a layer—and explain why

Breaking a subject into a few cards is easy. Explaining why those particular cards belong together takes more thought.

Suppose you are organizing a course on user interviews. You write down “prepare questions,” “recruit participants,” “open questions,” “reflection,” and “confirmation bias.” All five belong to the subject, but they use different organizing principles. Some are steps, one is a technique, and one is a risk.

Decide what this layer should explain

Before dividing a subject, give the layer a purpose in one sentence. If it answers “How do I run an interview?”, a sequence makes sense: prepare, conduct, organize, reflect. If it answers “How do I improve interview quality?”, you might instead organize it around asking, listening, recording, and judging.

Either approach can work. Your choice depends on what you want the map to help you understand.

A layer’s organizing principle gives the reader a way to read it.

Separate the different dimensions

Once you choose to organize by process, “open questions” can sit under “conduct the interview.” Confirmation bias may affect several stages. Give it its own place and connect it to the relevant cards.

The reader can now see a clear process before entering the details. Each idea has a place, without crowding the overview just to draw attention to it.

The branches do not need to be equally deep. One step may need three layers of explanation; another may need only a sentence. Depth should follow the needs of the explanation.

Check for gaps in two ways

After arranging a layer, try these checks:

• Is an important part missing? Follow the organizing principle you chose and look for material that still has no place.
• Has another dimension slipped in? Explain why each card belongs alongside the others. If you keep changing your criteria, the cards may need regrouping.

Neither check guarantees a complete map. Both give you a reason for the next change. You can tell whether you are filling a gap or changing how the whole subject is organized.

Let the divisions change as you learn

On a first reading, the author’s table of contents is often a useful starting point. Later, you may find that the question you care about crosses several chapters.

Keep the material and its citations, and move the cards into a structure that better serves that question. Leave yourself a short explanation of the change. When you return, you will still know why this layer is divided the way it is.