Why is the price elasticity of demand defined in term of percentage changes, rather than changes?

Answer 1

Since price elasticity takes into account both changes in price and quantity, it would be like comparing apples to oranges if it weren't converted into percentage change.

Answer 2

The previous answer is correct, but more generally, you want your answers to be as independent of units as possible.

For example, when you switch from pounds to tons, the percentage change is the same, but the change in absolute numbers would be different. Ditto just about every other number.

Also, percentages are usually (though not always) more meaningful. A profit of 10% is nice. A profit of $10/day is great if you are a kid with a lemonade stand (200% profit?), but pretty bad if you are a grocery store (0.1% profit?).

(One exception to the advantages of percentage figures is in terms of certain kinds of growth. A 10% growth for a tiny company is ho-hum. For the economy of China, it is major news. But even there, the absolute numbers alone aren't enough.)