A price change can produce a large or small response in quantity demanded. Price elasticity describes the size of that response using percentage changes, which lets us compare goods measured in different units.
In a simplified calculation, a 10% price increase paired with a 20% fall in quantity gives an elasticity of −2, often discussed using its magnitude of 2. The measurement depends on the market, time period and method. Observing sales after a price change also requires asking what else changed.