Data Dive 1.2

Can a chart make a small difference look huge?

Same numbers, two charts, two very different stories.

You will need paper and a pencil, and a browser for step 3

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Step 1 of 5 · Your guess

What do you think?

Los Angeles is split into 15 council districts. When something breaks, like a pothole, a streetlight, or trash on the sidewalk, people report it to 311, the city's help line.

Imagine a council member posts a bar chart on social media. Her district's bar is tiny next to the others. The caption says: “Our neighborhood is being ignored.”

Write it down before you lookHow many times more 311 requests do you think the busiest district has than the quietest one? 2 times? 10 times? 50 times? Write down a number.

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Step 2 of 5 · Look

311 requests in every district

Los Angeles, all of 2022. One bar is one district.

How to read it. Each bar is one council district. The taller the bar, the more 311 requests people in that district made in 2022.

Find out

Find the tallest bar and the shortest bar. Write down both numbers.
Does any district look ignored to you?

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Step 3 of 5 · Try it

Now you try it, on the same data

This opens the pool, the site's chart tool. The words in bold below are buttons and menus you will see on the screen.

Do these five things

Open the pool on this file: data.sfhsbta.com/thepool?file=bar_la_311_by_council_district.csv&mark=bar

Under across, pick council_district. Under up, pick requests. Set order to by value. Hover the tallest and the shortest bar and write down both numbers. Divide the big one by the small one.
Find axis starts at and change it from zero to min. Look at the chart again. How many times taller does the tallest bar look now?
Did any of the numbers change?
Which chart would the council member post? Which one is fair?
Open bar_quakes_by_magnitude_band.csv, the earthquakes sorted by strength. Divide the tallest bar by the shortest. Compare that with your answer from step one.

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Step 4 of 5 · Say it

Your answer, in one sentence

I think the district is <ignored or not ignored> because <what the chart shows> and I checked it by <what you did to the chart>.

The last part matters most. It shows you did not just look at the chart, you tested it.

Here is one exampleI think the district is not being ignored because the busiest district has only 1.6 times as many requests as the quietest. I checked by starting the axis at min, and the same numbers suddenly looked like a huge gap.

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Step 5 of 5 · What you found

Check your numbers

1.6x

the busiest district compared with the quietest

district 9

the busiest; district 5 is the quietest

1,684x

the most common earthquake size compared with the rarest

numbers that changed when the axis moved

The big idea: a cut axisA bar's height is supposed to be its number. When the axis starts above zero, the bars stop matching their numbers and small differences look huge. Always check where the axis starts.

One more thing to think aboutThe earthquake chart and the district chart are the same kind of chart. One has a 1.6x difference and one has a 1,684x difference. Would you have known that without reading the numbers?

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After the dive

The charts you used today

the bar

Bar length is the value, so it only means anything measured from zero. Start the axis anywhere else and the lengths stop matching the numbers.

What it leaves outOrder is a choice. Sorted by size it is a ranking; sorted by name it is a lookup; and the same bars say different things in each.

library · ladder · dives · the pool · this dive’s worksheet

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