Often, you will want to compare the differences between measured values across different categories. You might find yourself asking the following questions:

• How many customers did each store serve?
• How much energy did each wind farm produce?
• How many patients did each doctor see?

In each case, you are looking to make a comparison (among stores, wind farms, or doctors) in terms of some quantitative measurement (number of customers, megawatts of electricity, and patient).