Percentage Change and Percent Difference
When looking at a table or a graph, it is useful to understand the meaning between
.
changes in different data points. For instance, if you are looking at a table or
graph that is broken up into time periods, and there is an increase or decrease over
time, how do you describe that change? There are two methods and both of them
can be useful when trying to describe trend data.
First, understanding the difference between the two is very important. Let’s say
that a small auto dealership sells on average 2 cars per day. One day the
dealership sells 6 cars. This is an increase of 4 cars, or 200%. The larger
dealership across the street sells on average 10 cars per day and sold 14 cars that
same day. Here the larger dealership had the same increase in cars sold as the
small dealership but it only increased its car sales by 40%. So even though the
larger dealership sold more in total it didn’t increase its sales by more than the
small dealership.
Percent Difference
One method for presenting data is percent difference. This is a basic subtraction
of one data point from another. By taking the two points and subtracting the old
point from the new, you find the difference between them. If the data points are
decreasing, the calculation will produce a negative number. When interpreting the
change, all that is needed is the absolute value of the difference. An absolute
value is found by simply removing the minus sign and turning the negative
number into a positive number. Here are two examples that illustrate this.
1. If in 2003 the number of adults in Minnesota, ages 18 thru 24 who reported
using alcohol within the past 30 days was 46.1% and in 2006 it was reported
that 70.1% of those same aged adults in Minnesota used.
70.1 – 46.1 = 24.0
The percent difference is 24.0%. This is to say that between the years 2003-2006,
the percent of adults in Minnesota, ages 18 thru 24 who reported using alcohol
within the past 30 days had increased by 24.0 percentage points.
