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Comment by nonbel | Hacker News Reader

>"I got very grumpy at an official graph of British teenage pregnancy rates that apparently showed they had declined to nearly zero. Until I realized that the bottom part of the axis had been cut off, which made it impossible to visualize the (very impressive) 50% reduction since 2000."

Don't understand. I'd think not truncating the y-axis would make it more difficult to see a change.

>"I thought people would know that 3 out of 100 is equal to 3% is equal to 0.03. But they are very different!"

Don't understand. Those numbers are all the same by definition.

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> >"I thought people would know that 3 out of 100 is equal to 3% is equal to 0.03. But they are very different!"

> Don't understand. Those numbers are all the same by definition.

They are ways of representing that same number but people respond to them differently.

So the numbers are not very different, just erroneously perceived to be different (according to psychologists). I guess it isn't as odd as I thought after skimming, but I still don't like it.

Numbers are not different mathematically. Written representations of numbers are different psychologically.

If you start a graph at like 50 on the y axis it's easy to make data look more pronounced. The official graph showed they dropped to "zero", presumably a bigger difference than 50%. Like it would look like the line dropped to zero, but really the graph started higher than zero on the y axis to begin with.

Idk about the second one.

Thanks. I didn't think that he meant they were exaggerating the change, but you seem to be correct.

A good guideline for whether to truncate a y-axis or not is that the conclusion that a reasonable and knowledgeable researcher would draw after looking at the data should be clearly implied based on a quick cursory glance at the chart. If the chart implies an unreasonable conclusion based on a cursory glance, then it might be said to be misleading. A chart's purpose is to convey information quickly, not to convey information precisely -- for that, a table is often better.

In the example, the true effect is that there was a ~50% decrease, which is newsworthy and interesting in and of itself. Yet, intentionally or not, the chart implies a ~100% decrease based on a quick glance, and so fails to highlight the conclusion that a reasonable person would draw from the underlying data. In this case, I would say a y-axis from 0-60% would be appropriate.

See this: https://twitter.com/d_spiegel/status/707860293683126272

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