Shift vs. Movement Along the Curve: What's the Difference?
The Core Rule
A movement along a demand or supply curve is caused by one thing and one thing only: a change in the price of the good itself. The curve stays exactly where it is; you simply slide to a different point on it. A shift of the curve is caused by anything other than the good's own price — income, tastes, input costs, technology, taxes, expectations, the number of buyers or sellers. The whole curve relocates left or right, meaning that at every possible price, the quantity is now different. If you can state the cause of the change and it is not the good's own price, you are looking at a shift.
The Vocabulary Trap
Textbooks use different words for the two cases, and exams test whether you know them. A movement along the demand curve is a change in quantity demanded. A shift of the demand curve is a change in demand. Likewise, a movement along the supply curve is a change in quantity supplied, while a shift is a change in supply. Writing "demand went up" when you mean "quantity demanded went up" is not a stylistic slip — it describes a completely different event with a different cause and different consequences. When an exam question says "demand increased," it is telling you the whole curve moved right.
Worked Example: Coffee
Suppose a frost destroys part of the coffee harvest. The frost raises production costs, so the supply curve shifts left. The equilibrium price of coffee rises. Buyers respond to that higher price by purchasing less — but that is a movement along the demand curve, not a shift of it, because the only thing that changed for buyers is coffee's own price. Now suppose instead that a study links coffee to health benefits. That changes tastes, so the demand curve shifts right, price rises, and sellers respond by producing more — a movement along the supply curve. In each case one curve shifts and the other is merely traversed.
A Quick Diagnostic
Ask three questions in order. First: what changed? Second: is the thing that changed the price of this exact good? If yes, it is a movement along a curve. If no, it is a shift. Third: whose behavior did it change — buyers or sellers? Income, tastes, related-good prices, buyer expectations, and the number of buyers shift demand. Input prices, technology, taxes and subsidies, seller expectations, and the number of sellers shift supply. Answer those three questions and you can draw the diagram correctly before you have written a word of analysis. The remaining work — reading off the new equilibrium — is mechanical.