Data Dive 1.4

What is the most common tree on LA streets?

The city keeps a list of every spot on its streets where a tree grows, or could.

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?

Picture the streets of Los Angeles. On postcards, it is all palm trees.

The city keeps a list of every spot along its streets where a tree is planted, or where one could be. There are hundreds of thousands of spots on that list.

Write it down before you lookWhich tree do you think is the most common? Write down your top three guesses.
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Step 2 of 5 · Look

The city's list, biggest first

The Los Angeles street tree list, biggest entries firstdata.lacity.orgstreet spotsentry on the liststreet spots58,360116,7200116,720Vacant - Ok To Plant45,000Crape Myrtle42,013Mexican Fan Palm39,095American Sweetgum33,955Southern Magnolia29,914Indian Laurel Fig21,042Jacaranda18,351London Plane17,092Camphor Tree15,645Canary Island Pine

The ten biggest entries on the Los Angeles street tree list.

How to read it. Each bar is one entry on the city's list. The longer the bar, the more street spots have that entry.

Find out
  1. Read the top bar. Is it a tree?
  2. Find the palm. Were your guesses right?
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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=picture_la_street_trees_by_species.csv&mark=pictograph

  1. Under across, pick species. Under up, pick sites. This is a picture chart: each icon stands for many street spots. Read the biggest entry. Is it a tree?
  2. Set top to 10. How many of the ten are not a kind of tree?
  3. Click the bar shape and find the Mexican fan palm. What place is it in?
  4. Click the pictograph shape again. It only draws the ten biggest entries. The file has 20 rows. How many are not drawn at all?
  5. Should “VACANT - OK TO PLANT” be on a chart of trees? Decide, and be ready to say why.
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Step 4 of 5 · Say it

Your answer, in one sentence

I think the most common tree on LA streets is <a tree name> 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 most common tree on LA streets is the crape myrtle, because its bar is the longest one that is actually a tree. I checked by switching to a bar chart and skipping the top entry, which is empty spots.
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Step 5 of 5 · What you found

Check your numbers

116,720
the biggest entry: empty spots, not trees
crape myrtle
the most common real tree; the palm is third
10 of 20
entries the picture chart does not draw at all
The big idea: the shape decides what you seeA picture chart only has room for a few icons, so it quietly drops the small groups. The data did not change. The shape of the chart did. Before you trust a chart, ask what it had no room for.
One more thing to think aboutThe top entry is a spot where a tree could grow but none does. There are 116,720 of them. Where in the city do you think those empty spots are? What would you want to open to find out?
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After the dive

The charts you used today

the picture graph

One little picture per thing, lined up so you can count them.

What it leaves outCounting stops working somewhere above a couple of dozen, and a row too long for the page gets cut rather than shrunk.
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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