The Bar Is 1.58 Times Taller. On Screen It Looks 4.5.

You'll learnto read and build a bar chart for one categorical variable, and to spot the three ways the picture can say something the counts do not.

Four hundred students picked a lunch. Pizza took 152 and salad 96 — a ratio of 1.58 to 1, and a bar chart draws that ratio to scale when the value axis starts at zero. Start that axis at 80 instead and the two bars become 72 and 16 units of ink: pizza now looks four and a half times salad, an exaggeration of 2.84, with every number on the chart still correct. This animation builds the chart, truncates it, then tries to put a multi-select survey into a pie and watches the slices run past a full turn at 187 percent. It finishes on the difference between a bar chart and a histogram, using two sets of twenty scores whose histograms are identical and whose means are five points apart.

8 STEPS · 6 QUICK CHECKS · BAR CHART · TRUNCATED AXIS · PIE AND ITS WHOLE · NOT A HISTOGRAM · v1

ratio 1.58 drawn as 4.50 · a pie at 187% · identical bins, means 5.1 apart

Before you start

What you're looking at

400 students, one categorical variable — which lunch they picked — drawn as a bar chart with one bar per category.

The question

Every number on these charts is correct. So what makes one of them tell the truth and another one not?

Watch for

The axis starting at 80, the pie slices running past a full turn, and the two data sets whose histograms cannot be told apart.

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