# Section 3: Percent Change

## Measuring how things change

Finally, there is a measurement that we often make when we’re comparing the results of some sort of experiment. We want to know how much things changed as a result of whatever we did – if I add fertilizer, how much taller does my plant grow? If I feed my mouse less, how much longer does he live? And so on. Or, we might just want to know how things change over time – such as, how did the price of gasoline change from 1998 to 2008?

To look at how things change, we use percent change. Here’s how it works. Let’s say I want to know the percent change in the price of a gallon of gas over the course of the last decade.

• 1998: \$1.40
• 2008: \$4.20

First I calculate the actual change, which is

\$4.20 - \$1.40, or \$2.80

Then I compare the change in price to the baseline price – that is, the price at the beginning of the timeperiod. So my ratio looks like this:

change in price : baseline price = \$2.80 / \$1.40

= 200%

### Likewise I could compare the percent change in fat in the Old-Fashioned Pound Cake vs. the Lowfat Pound Cake.

(To make this problem interactive, turn on javascript!)

• I need a hint: Now the Old-Fashioned Pound Cake is the baseline
• ... another hint ... : change in butter = 13 oz – 16 oz = -3 oz
• ... another hint ... : change in butter : baseline butter = -3 oz / 16 oz

#### I think I have the answer: -19%

The minus sign tells me that the amount went down compared to the baseline.