Do I always need a diagram in economics, and how much is it actually worth?

Draw one whenever the question involves a change in a market or in the wider economy — which is most analysis questions. But the diagram itself earns little. The marks are for the explanation that walks through it, and a correct diagram sitting unreferenced beside the text wastes the time it took to draw.

What a diagram has to have

Four things, and losing any of them costs marks that have nothing to do with economics:

Both axes labelled, with units. Price and Quantity for a market diagram; Price Level and Real GDP for a macro one. Writing P and Q without saying of what is a common slip.

Curves labelled, and labelled again after they shift — S to S₁, not two unnamed lines.

The original and new equilibrium marked, with dotted lines down to the axes.

A title or a sentence saying which market this is. "The market for coffee beans" takes three seconds and prevents the examiner guessing.

Then explain the movement in steps

This is the part that scores, and it should read as a sequence:

1. Name the event and which curve it shifts, and why. "A poor harvest reduces supply, shifting S left to S₁, because production costs rise and output falls."
2. Say what happens at the old price — a shortage or a surplus.
3. Explain the adjustment. "Excess demand puts upward pressure on price."
4. State the new equilibrium and the direction of change in both price and quantity.

Four steps, usually four marks. Candidates who write "supply falls so price rises" have given the first and last and skipped the mechanism in between, which is where the analysis marks live.

The distinction that catches people out

A shift versus a movement along. Only a change in a determinant other than price shifts a curve. A change in price causes movement along it.

So a tax on producers shifts supply. A rise in the price of the good itself does not — it is the consequence, not the cause. Drawing a shifted demand curve in response to a price change is one of the most heavily penalised errors in the subject, because it shows the model has not been understood.

If you find yourself shifting a curve because the price changed, stop and reconsider which event the question actually described.

Elasticity changes the answer

A question specifying that demand is inelastic is telling you to draw it steep, and the shape determines the result.

With inelastic demand, a supply shift produces a large price change and a small quantity change. With elastic demand, the reverse. Questions about agricultural markets, fuel, or addictive goods are usually built on this, and the whole point is lost if the curve is drawn at a generic forty-five degrees.

Say what you have drawn and why: "demand for insulin is price inelastic, shown by the steep D curve, so the supply reduction raises price sharply while quantity falls only slightly."

When not to bother

A purely definitional question, or one asking for evaluation of a policy's practical difficulties, does not need a diagram and the time is better spent writing. Judgement about when to draw is itself part of exam technique — a beautifully drawn diagram on a question that did not want one has cost you minutes you needed elsewhere.