The "which display is inappropriate" item

Three pies from one quarter

The marketing lead at Maple & Birch sends three charts for the quarterly deck. Each is a pie, each looks finished, and each is the "which display is inappropriate" item from the midterm.

Pie 1 — revenue by product, twelve slices. Twelve SKUs, from the office chair at 22 % of revenue down to a box of paper clips at 1 %. The eye cannot rank slices: a 9 % wedge and an 11 % wedge look identical, the small ones need a legend with twelve colours, and the labels overlap at the thin end. The rule of thumb across the OpenStax texts is five or six slices at most. Replace it with a sorted bar chart: twelve bars, longest at the top, every value readable, no legend needed.

Pie 2 — "why customers chose us", slices summing to 140 %. The survey let each customer tick every reason that applied: price 60 %, delivery 45 %, range 35 %. A pie divides one whole into parts, so its slices must be mutually exclusive and sum to 100 %. Multi-select answers overlap — the same customer sits in two slices — and the chart software either scales the 140 % down to a full circle (silently changing every number) or draws nonsense. The fault is not rounding and not the data; it is that these categories are not parts of one whole. A bar chart with one bar per reason, each reaching its own percentage, tells the truth.

Pie 3 — two pies, channel mix 2024 and 2025. Online was 44 % of a 2.4 M year and is 50 % of a 3.0 M year. Two pies side by side ask the reader to compare a wedge angle in one circle with a wedge angle in another, and to remember that the circles represent different totals — which a pie cannot show at all. Comparison across periods is a bar question: a clustered column (Online and Store, 2024 beside 2025) if the amounts matter, or a 100 % stacked column (one column per year) if only the mix does.

The one legal pie

A pie is acceptable when the data is parts of a single whole, the parts are mutually exclusive and exhaustive, and there are few of them — two to five or six. Maple & Birch's channel split, Online 49.8 % and Store 50.2 %, qualifies. Even then a bar chart loses nothing, and the quiz never penalises the bar.

Three-D pies, exploded slices and gradient fills are the same chart with the numbers made harder to read: the tilt of a 3-D pie makes the front slices look larger than the back ones. The fix for a bad pie is never a fancier pie.

Name the fault, then the substitute

The questions each show a display and ask what is wrong with it and what replaces it.

CRISP-DM: choosing an honest display is evaluation → review process applied to the deck before it ships.