Percent change = ((New Value − Old Value) ÷ Old Value) × 100%
If Old Value is 0, percent change is undefined (or not meaningfully comparable)
Increase: result is positive
Decrease: result is negative
Example: ((50 − 40) ÷ 40) × 100% = 25%
Example: ((30 − 45) ÷ 45) × 100% = −33.33%