Difference method

A difference dataset is simply the difference between values for each location in two specified datasets at two specified times.  The easiest way to create a difference dataset is using the "Difference" method (Method -> Difference), as described here.  You can also calculate differences using the "Derive new dataset" option from the Tools pulldown menu (or from the right-click menus for geographies) following instructions shown here.  Note one key difference between the two ways to calculate difference -- for the difference method, you need to specify times, even for datasets (i.e., males and female cancer rates) where you have data for many times.  If you create a derived dataset from the tools menu, you can subtract datasets without specifying a time, and thus you can explore how the difference between datasets changes over time.

Here are two ways that difference datasets are commonly used:

• To examine how values for different variables vary at a given time. For example, you can use the method to examine differences in lung cancer rates between women and men.

• To observe how one dataset changes through time. For example, you could analyze how lung cancer incidence rates differ across years, as in the example below.  Note in this example, rates in all polygons were higher in 1980 than in 1960, so the difference is always positive.  In many cases, you will have both negative and positive values in your difference map.

For step-by-step information on how to use the difference method to create a difference dataset and a difference map, click here.