Q.Distinguish between Movement along the Demand Curve and Shift of the Demand Curve.

Both concepts describe a change in the quantity demanded, but they are caused by two completely different kinds of factors and must never be confused.

A movement along the demand curve happens when quantity demanded changes only because of a change in the commodity's own price, with every other determinant held constant, and it stays on the same demand curve. Extension of demand is a rise in quantity demanded caused by a fall in price (a movement downward and to the right along the curve); contraction of demand is a fall in quantity demanded caused by a rise in price (a movement upward and to the left along the curve).

A shift of the demand curve happens when quantity demanded at every given price changes because of a change in a non-price determinant — income, tastes, the price of a related good, population, expectations, or government policy — while the commodity's own price stays unchanged, moving the buyer onto an entirely new curve. Increase in demand is a rightward shift of the whole curve (buyers demand more at every price), caused, for example, by a rise in income, a rise in a substitute's price, or a favourable change in taste. Decrease in demand is a leftward shift of the whole curve (buyers demand less at every price), caused by the reverse changes. …

Students often use 'increase in demand' loosely to mean any rise in quantity demanded, without checking whether the cause was the commodity's own price — which would actually make it an 'extension,' not an 'increa …