Defining new variables

When given a data set to analyse, think about whether its variables are the most useful ones to analyse. Sometimes a simple transformation provides a variable whose values are more meaningful or highlight a different aspect of the data.

For example, it is usually more meaningful to compare the population density of countries than either their populations or areas:

Density   =   Population   × 1000
Area

As another example, although it is interesting to compare the calorie intake of countries, it is also appropriate to look at their percentage change over different periods:

%Increase   =   100  ×   2009 calories − 2004 calories
2004 calories