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
Likewise I could compare the percent change in fat in the Old-Fashioned Pound Cake vs. the Lowfat Pound Cake.
- 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.
Copyright University of Maryland, 2007
You may link to this site for educational purposes.
Please do not copy without permission
requests/questions/feedback email: firstname.lastname@example.org