Data Dive 1.5

Is half of 311 really about old couches?

A donut chart answers one question well. Try it on a second one.

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?

In Los Angeles, you can call 311 to ask the city to pick up big junk left on the curb: couches, mattresses, old fridges. The city calls these bulky items.

Someone says bulky items are half of everything 311 does.

Write it down before you lookWhat share of all 311 requests do you think is bulky items? Write down a percent.

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Step 2 of 5 · Look

What people ask 311 to do

Los Angeles 311 requests by type, 2022. Here they are bars. In the pool you will see them as a donut.

How to read it. Each bar is one kind of request. The longer the bar, the more times people asked for it in 2022.

Find out

Find bulky items. Is it about half of everything?
Which kind of request comes second?

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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=donut_la_311_by_request_type.csv&mark=donut

Under across, pick request_type. Under up, pick requests. You will see a donut. Without hovering, put the three middle-sized slices in order, smallest to biggest. Write your order down.
Set view to table and check your order. How many did you get right?
Go back to the chart. In the key, press Bulky Items to hide it. What does the rest of the city's work look like now?
Some slices are so thin you cannot see them. Hover around the donut to find them. How many are there?
Write one thing a donut chart is good at, in one sentence.

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Step 4 of 5 · Say it

Your answer, in one sentence

I think the claim 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 claim is almost right, because bulky items are 46.5% of all requests, just under half. I checked by hiding bulky items in the donut, and graffiti became the biggest slice of what was left.

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Step 5 of 5 · What you found

Check your numbers

46.5%

bulky items, just under half

24.4%

graffiti, the next biggest

kinds of request so small the donut draws nothing

The big idea: what a donut is good forA donut is good at one job: showing that one thing is a big part of the whole. It is bad at comparing slices that are close in size, and it hides tiny ones completely. If you need to rank things, use bars.

One more thing to think aboutThe three middle slices are 7.9 / 7.5 / 4.5%. Did your eye put them in the right order? What does that tell you about charts made of angles?

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After the dive

The charts you used today

the donut

Slices add up to the whole thing. Good for parts of one total, bad for ranking.

What it leaves outSlices close to the same size cannot be told apart, and one large slice flattens every other into a sliver you cannot read.

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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