The midpoint method calculates price elasticity using the average of the starting and ending prices and quantities.
For a price change, the method divides the percentage change in quantity by the percentage change in price, with each percentage change measured relative to the average of the two endpoints. This is also called the arc-elasticity approach.
Using averages avoids obtaining different numerical elasticities depending on whether the calculation starts at the original point or the new point. That symmetry makes the method useful when analyzing a movement between two observed combinations of price and quantity.
The midpoint method differs from point elasticity, which measures responsiveness at a particular point and often uses calculus. Neither method automatically explains why demand changed; it only quantifies responsiveness between specified observations.