Chart traps that catch candidates
Cut-off axes, share versus absolute numbers, footnotes, units, and averages that hide the mix.
Key takeaways
- Before you trust what a chart seems to say, check the axis, the units, and the footnotes: most wrong answers come from skipping these ten seconds.
- Common mistakes: Reading bar heights instead of the numbers.
- Cut-off axis: the axis starts at 90 instead of 0, so a small difference looks huge.
- Share versus absolute: a falling share can hide growing sales, and a rising share can hide falling sales when the market shrinks.
- Units and footnotes: thousands or millions, local currency or US dollars, "excludes online sales", "at constant prices".
Key idea
Before you trust what a chart seems to say, check the axis, the units, and the footnotes: most wrong answers come from skipping these ten seconds.
The six traps
- Cut-off axis: the axis starts at 90 instead of 0, so a small difference looks huge. Read the numbers, not the bar heights.
- Share versus absolute: a falling share can hide growing sales, and a rising share can hide falling sales when the market shrinks.
- Units and footnotes: thousands or millions, local currency or US dollars, "excludes online sales", "at constant prices". One footnote can change the answer.
- Percent versus percentage points: a margin going from 10 to 12 percent is up 2 points, which is 20 percent growth.
- Averages hide the mix: the average price can fall only because customers bought more of the cheaper product, even if no price changed.
- Different periods: a quarter compared with a full year, or a year with an extra month.
Worked case
Sales up, share down
The prompt
A table shows our sales in Europe rising from EUR 100 million to EUR 120 million, while the market grew from EUR 400 million to EUR 600 million. The client says: "We are doing great." Are they?
The structure
- Compare absolute growth with share
- Our growth
- Our share before and after
Working it through
1. Our growth
From 100 to 120.
Our growth (percent):(120 - 100) ÷ 100 × 100 = 202. Share before
100 out of 400.
Share before (percent):100 ÷ 400 × 100 = 253. Share after
120 out of 600.
Share after (percent):120 ÷ 600 × 100 = 20
The recommendation
No, the client should not call this great performance, because sales grew 20 percent while the market grew 50 percent. As a result, share fell from 25 percent to 20 percent, a loss of 5 percentage points, which means competitors are growing faster. The risk is that the gap keeps widening if the market's growth comes from segments or channels where the client is weak. As a next step, split market growth by segment and channel to find where competitors are winning.
A bar chart's axis starts at 90. One bar shows 96, the other 100, so the second bar looks almost twice as tall (about 1.7 times). What is the real difference, in percent? (One decimal place.)
Reading bar heights instead of the numbers. Ignoring a footnote such as "excludes online". Saying "grew 2 percent" when a share moved 2 points. Missing that one exhibit is in thousands and the other in millions.
A chart's axis starts at 50, not 0. What should you do?
Average price per unit fell, but every product kept the same price. How is that possible?
A footnote says "excludes online sales". Why does it matter?
Sources for this lesson (1)
- Recognized public explanations of case-interview concepts and frameworks
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