Percentage change = (x2 - x1) / x1
That midpoint thing should produce different results. Ex: Gas goes from $4 to $10 (change of 150%), and demand drops 10%. So we have elasticity of 1/15. Using midpoint, the 10% drop becomes 10.53%, and the 150% gain becomes an 85.7% gain, for elasticity of about 8.14.
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