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.
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