Table of Contents

• Prequiz
• 1: Three tricks division can do
• Section 1: Ratios and Fractions
• 2: Finding Ratios
• 3: More uses for Ratios
• Section 2: Proportions and Percents
• 4: Proportions and Percents: Parts of the Whole
• 5: Absolute amounts from percents
• 6: Parts and wholes
• Section 3: Percent Change
• 7: Measuring how things change
• 8: Percent Change is not a direct comparison
• 9: Units, units, units
• Section 4: Biology
• 10: What's in your milk?
• 11: An experiment

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%

 

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