Data Dive 3.5

Does most of California breathe bad air?

A shaded map of the state looks alarming. Look closer.

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?

Someone holds up a map of California. Each county is shaded by its air quality, and most of the map is dark.

They say most of California breathes bad air.

Write it down before you lookIs that true? Before you look, write what you think the map is shading: people, or land?

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Step 2 of 5 · Look

Air quality in California's biggest counties

The 22 counties with the most people, 2024.

How to read it. Each bar is one county. Its height is the county's typical air quality index, or AQI. Higher means worse air.

Find out

Which counties have the worst air?
On a map, which county would take up the most space? Is it the one with the most people?

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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=pair_ca_counties_area_population_aqi.csv&mark=bar

Under across, pick county. Under up, pick land_sq_mi, the land area in square miles. Set order to by value and top to 10. Write down the ten biggest counties.
Under up, pick population_2024. Set top to 10 again. Write down the ten counties with the most people.
How many counties are on both lists?
Find San Bernardino and Los Angeles on each list. Which has more land? Which has more people?
Someone is deciding where to put an air monitor. Which chart would you send them?

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Step 4 of 5 · Say it

Your answer, in one sentence

I think most of California <does or does not breathe bad air> 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 dark map is misleading, because the 45.7% of the land in the emptiest counties holds only 4.9% of the people. I checked by switching up from land to population, and most of the top ten changed.

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Step 5 of 5 · What you found

Check your numbers

4 of 10

counties on both top-ten lists, land and people

45.7%

of California's land is in its 29 emptiest counties

4.9%

of its people live there

23%

San Bernardino's people, compared with LA's

The big idea: a shaded map colors land, not peopleA shaded map gives the most color to the biggest counties, even if hardly anyone lives there. Land does not breathe. People do. If a question is about people, check a chart of people, not of area.

One more thing to think aboutImagine a map where every person got the same amount of space. Would you still recognize California?

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After the dive

The charts you used today

the choropleth map

Darker means more, lighter means less — one number, shaded region by region. Big regions grab the eye whether or not many people live there.

What it leaves outBig areas shout. A large empty region looks more important than a small crowded one.

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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