Why do economics use percentage cgange to calcuate elasticity demand?

Question

Why do economics use percentage cgange to calcuate elasticity demand?

Answer 1

We focus on percentage change because we are interested in the effects of a price change.  When a company changes the price of its goods it is interested in the change in quantity demand because the change is, presumably, caused only by the change in price.

Answer 2

Percent changes are unit free and give a better picture of how consumers respond to a price change.  Think of changing the price of a good by $1.  If we are talking dollars, then the unit change in price is one unit.  However if we are talking pennies, then the same $1 change would show up as a 100 unit price change (100 pennies in a dollar).  Using percentage change takes this issue off the table.