Data Dive · Rung 2 · What's Reported · 2.4

The day Los Angeles went quiet

The hole and the spike are the same event, and neither of them is people.

Runs in 25-40 minutes · everything you need is on this page · no network, no accounts, nothing for the students to open

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1 — open cold

Put this up. Say nothing about it.

Los Angeles 311 requests per day, 2022every service request the city logged, one bar per day - data.lacity.orgrequests loggedmonth of 20223,0846,1680010203040506070809101112

Los Angeles 311 -- requests logged per day, all of 2022

Every bar is one day. The city averages 3,638 requests a day.

AskWhat is this? Take answers until somebody names the axes.
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2 — the hole

Now point at the gap.

Los Angeles 311 requests per day, 2022every service request the city logged, one bar per day - data.lacity.orgrequests loggedmonth of 2022every other daythe outage weekend3,0846,1680010203040506070809101112

Same chart. Two days marked.

95
requests, Sat 24 Sep
3,638
the average day
3,716
next day, busiest Sunday of 2022

Do not correct anything yet. You will get a holiday, rain, an event, people were busy. Every one of those is about people. Write them where the class can see them.

Ask -- and write every answer on the boardWhat happened that weekend?
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3 — the reveal

Same weekend. Now by the hour.

311 requests per hour, 23-25 September 2022the same weekend, counted by hour instead of by dayrequests loggedhour, across three daysFriday and SundaySaturday 24 September157.5315009-2309-2409-25

Every hour, Friday 23rd to Sunday 25th September 2022. Saturday in yellow.

19
consecutive hours at zero
21:55
last request Friday
19:49
first request Saturday

It is not. The 311 system was down. And the Sunday spike stops being a busy day — it is the backlog landing.

AskNobody reported a single thing for nineteen hours, then everybody started again at a quarter to eight in the evening. Is that people?
Go back to the boardWhich of their explanations survives? Cross them out together. This is where it lands, and it lands because they were wrong first.
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4 — the sentence

One chart. Two features. One cause -- and the cause is not in the chart.

Anyone reading only the daily chart explains both with human behaviour, confidently, and is wrong twice.

It happened again on 26 March: seventeen empty hours, back at 7pm. Both outages end near 7pm on a Saturday. A maintenance window is the obvious guess and this data cannot confirm it — saying so is the right move.

The line to leave them withThe data does not show when things happened. It shows when they were recorded.
The three questions
  1. What was actually measured, and when was it recorded?
  2. What did the hourly chart show that the daily one could not?
  3. Name a number in your own life that records when something was entered, not when it happened.
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After the dive

The shapes on this page, in the library’s own words

the histogram

Bars touch because they are slices of one scale, not separate categories. Read the shape.

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

Follow it left to right. A dashed part is a guess about the future, not something anyone saw.

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.

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