Data Dive 4.1

Does a blank spot on a map mean clean air?

A report calls six counties clean. Find out why they look that way.

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?

A company wants to build a factory that will affect the air. Its report lists 6 California counties as good places to build, because on the state's air quality map they are blank, with no color at all.

The report says those counties have clean air.

Write it down before you lookWhat do you think a blank county on an air quality map means? Write it down.
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Step 2 of 5 · Look

Air quality by county, with the blanks marked

Median air quality index by California county, 2024the twenty-two most populous counties, and the six with no monitor - US EPA AirDatamedian AQIcountymedian AQI, measuredno air monitor43.5870Los AngelesSan DiegoOrangeRiversideSan BernardinoSanta ClaraAlamedaSacramentoContra CostaFresnoKernVenturaSan FranciscoSan JoaquinSan MateoStanislausSonomaTulareSolanoSanta BarbaraMontereyPlacerNapaYubaLassenModocSierraAlpine

California counties, 2024. The yellow bars are the blank counties.

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
  1. How tall are the yellow bars? Why?
  2. Does a zero here mean clean air?
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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=absence_ca_counties_air_monitoring.csv&mark=bar

  1. Under across, pick county. Under up, pick median_aqi. Under colour by, pick has_air_monitoring. Count the bars. The file has 58 counties. Where are the rest?
  2. Under up, pick population_2024. Count again. What color are the new bars?
  3. Hover the new bars. Write down how many people live in each, and add them up.
  4. Set keep to to 132,727, the number of people in Napa County. Count the bars of each color that are left.
  5. Would you sign the report? Be ready to say what you changed on the chart to decide.
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Step 4 of 5 · Say it

Your answer, in one sentence

I think the report <is right or wrong> 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 report is wrong, because the blank counties are blank since nobody measures their air, not because it is clean. I checked by putting population on the side, and bars appeared that were missing before, with 261,239 people living there.
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Step 5 of 5 · What you found

Check your numbers

6
of 58 counties have no air monitor
261,239
people live in them
19
counties with a monitor that have fewer people than Napa
52
bars you see with air quality on the side; the six are missing
The big idea: blank means nobody measuredA blank spot on a map does not mean nothing is there. It can mean nobody measured. Missing data is not good news or bad news. It is no news. Before you trust a blank, ask whether anyone looked.
One more thing to think aboutYou are choosing where to build. One county has middle-of-the-road air. One is blank. Which is safer to build in?
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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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