通过中点法计算弹性需求（ED）

Price elasticity calculations yield different results depending on direction because they use different bases for percentage changes. Moving from point A to B uses one base, while moving from B to A uses another. For example, a price change from $6 to $9 represents a 50% increase, but $9 to $6 is only a 33.33% decrease. This directional inconsistency creates conflicting elasticity values without a standardized calculation method.

The midpoint method calculates price elasticity by dividing percentage changes in quantity and price by their respective midpoints rather than starting points. This approach eliminates directional bias and produces consistent elasticity values regardless of movement direction. Using midpoints ensures a standardized base, making elasticity measurements comparable and reliable across different demand curve analyses.

Midpoint values are calculated by averaging the two points. For price, add both prices and divide by two: ($6 + $9) / 2 = $7.50. For quantity, add both quantities and divide by two: (120 + 80) / 2 = 100 units. These midpoints serve as the denominator when calculating percentage changes, ensuring consistent elasticity measurements regardless of which direction you move along the demand curve.

Using the midpoint method with a midpoint price of $7.50 and midpoint quantity of 100 units, the price change is calculated as (9 - 6) / 7.50 = 40%, and the quantity change is (80 - 120) / 100 = 40%. Both changes equal 40%, resulting in a price elasticity of 1. This consistency demonstrates the midpoint method's advantage over directional calculations that produce different elasticity values.

The directional approach yields elasticity of 0.67 moving from A to B and 1.5 moving from B to A. The midpoint method produces a consistent elasticity of 1 regardless of direction. This consistency makes the midpoint method superior for economic analysis because it provides a single, reliable measure of elasticity. Understanding degrees of elasticity of demand and the demand graph helps interpret these values in context.

Consistent elasticity measurements allow economists to accurately compare demand responsiveness across different products, markets, and time periods. Without standardization, conflicting elasticity values create confusion and unreliable economic analysis. The midpoint method provides this consistency by using a neutral base, enabling meaningful comparisons of how quantity demanded responds to price changes across various economic scenarios.

An elasticity value of 1 indicates unit elasticity, meaning quantity demanded changes by the same percentage as price. In the example, a 40% price change produces a 40% quantity change, resulting in elasticity of 1. This represents the midpoint between elastic demand (elasticity > 1) and inelastic demand (elasticity < 1), showing proportional responsiveness to price changes.