In this video, students will see different ways of computing a percentage change, and why one or another would be more appropriate to answer the question. Briefly, when comparing two values across time, then one is naturally the "base" -- you were 100 cm tall at age 9 and at age 10 you were 110 cm tall, therefore you grew 10% in your 10th year. In economics, we also use a concept called elasticity in which there is no natural base and so we employ a different base.