My brain is hurting this afternoon, because someone decided to use a stacked bar chart for two figures in the brand-new Georgetown Center for Education and Workforce report on employment and salary data for different undergraduate degrees, written by Anthony Carnevale and Ban Cheah. Here is the chart on unemployment data for different clusters of disciplines:
Stacked bar charts only make sense if adding the quantities make sense–that is, if each set of bars refers to the same population and also if sums are cumulative in some sense. So, for example, with a cohort, bar charts would make sense if you are piling up bars representing the proportion who finish high school, those who in addition finish two years of college, and then those who also finish an undergraduate degree. In this case, the bar chart is attempting to combine quantities for different populations (new graduates, more experienced graduates, and masters-degree recipients), and where there is no cumulative meaning to the sum of the unemployment percentages. While I don’t think this data requires a chart–something this simple really can be displayed as a table without pretty pictures–those who go for bar charts should use unstacked bars, such as the following:
Because of an earlier, erroneous version of the file (even more erroneous than the stacked-bar chart displayed above), I know that the stacked bar chart was created using Excel or another spreadsheet program, and someone evidently chose the pretty stacked bar icon. Ouch. This is a good report, and because it was clearly designed to be accessible for the general public, it really is a shame that the figures were formatted to be confusing rather than edifying.