“Ann, should my bar charts be horizontal or vertical?”
Ordinal variables get vertical bars
Ordinal variables follow a natural progression—an order. I arrange ordinal categories from left to right so my viewers can view the sequence across the page, which means their bars are vertical. Use vertical bar charts to display ages, salaries, and even and cohorts (e.g., the percentage of students from each graduating class who achieved x outcome).
Nominal variables get horizontal bars
Nominal categories—favorite ice cream flavors, types of organizations where conference attendees are employed—can be arranged in any order. I arrange nominal variables in a list from top to bottom, which means their bars are horizontal.
You can sort nominal variables from greatest to least or from least to greatest. Both are correct. My advice: Sort the data so that the item that warrants attention is displayed first—a high number deserving a celebration, or a lower-than-hoped-for number that needs to be turned around.
These are guidelines, not rules. If you can explain the logic for going against this guidance, then your graph is probably going to be alright. My goal is to develop critical thinking masterminds, not robots.
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