In this chapter, we are concerned with measuring how a change in one variable affects another. For example, we might be interested in how the equilibrium quantity traded responds to a change in the market price, or how demand responds to changes in consumer income. One issue with measuring quantitative changes is that different markets use different units of measurement (litres, kilograms, ounces and so on), each market has its own price level (a few cents or millions of dollars) and it is even possible that different markets use different currencies. A way we can compare quantitative changes across different situations is to look at proportional (or percentage) changes. 1