Data Dive 2.4

Did all of LA stop reporting problems for a day?

One strange Saturday, and what really happened.

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?

311 is the number people in Los Angeles call for potholes, broken streetlights, or trash. On an average day, the city logs about 3,638 requests.

On Saturday, September 24, 2022, it logged 95. The next day was the busiest Sunday of the year.

Write it down before you lookWhat do you think happened that weekend? Write down every explanation you can think of.
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Step 2 of 5 · Look

Every day of 2022

Los Angeles 311 requests per day, 2022one bar per day - data.lacity.orgrequests loggedmonth of 2022every other daySaturday and Sunday, 24 and 25 September3,0846,1680010203040506070809101112

Los Angeles 311, all of 2022. One bar is one day. The weekend is marked.

How to read it. Each bar is one day. Its height is how many requests the city logged that day.

Find out
  1. Find the yellow bar in September, and the gap right before it. What do you see?
  2. Can this chart tell you what happened during that Saturday?
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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=hist_la_311_per_day.csv&mark=dotplot

  1. Under up, pick requests. This is a dot plot: one dot per day. Hover the lowest dot. Write down its date and its number.
  2. Set keep to to 500. How many days are left? Hover the other one. What date is it?
  3. Clear keep to. Set keep from to 6,000. How many days are left? Is the Sunday after the quiet Saturday one of them?
  4. Open hourly_311_outage_2022_09_24.csv and click the line shape. Under across, pick hour. Under up, pick requests. Count the hours in a row with zero requests.
  5. Look back at your list of explanations. Cross out every one that your count in step four rules out.
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Step 4 of 5 · Say it

Your answer, in one sentence

I think that weekend, <what happened> 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 311 system was broken that weekend, because nobody logged a single request for 21 hours and then everyone started again at once. I checked by opening the hourly file as a line and counting the zeros.
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Step 5 of 5 · What you found

Check your numbers

95 on 2022-09-24
the quietest day of the year
21
hours in a row with zero requests
Sat 19:00
when the requests came back
2
days under 500; the other is 26 March
The big idea: when it was recorded, not when it happenedData often shows when something was written down, not when it happened. The problems did not stop that Saturday. The system that records them did, and Sunday's spike was the backlog. When a chart has a strange gap, ask whether the counting stopped.
One more thing to think aboutThe other quiet day, 26 March, also came back on a Saturday evening. What might a city do on Saturday evenings that could turn a system off? How could you find out?
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After the dive

The charts you used today

the histogram

The bars touch because they’re slices of one scale, not separate things. Step back and read the shape — where the data piles up, and where it thins out.

What it leaves outThe bin width is a decision, not a fact. Widen the bins and two humps become one; narrow them and noise starts to look like structure.
the line

A line follows one thing over time. Read it left to right and watch which way it leans — that lean is the whole story. A dashed stretch is a guess, not something anyone measured.

What it leaves outThe line implies every point between the dots, and nobody measured those. Where you start and stop it decides what the trend looks like.
the dotplot

One dot per thing, stacked up wherever it lands. You can still count every dot, and the piles show where they gather.

What it leaves outAbove a few hundred values the dots run out of room and start to overlap, so the stack stops being an honest count.

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

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