Data Dive 3.2

Was September a normal month for LA's 311?

A monthly report says yes. You might know better.

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 dive 2.4, you found a Saturday in September 2022 when LA's 311 system went down and logged only 95 requests.

Now a monthly report says September was an ordinary month: 6th busiest out of 12.

Write it down before you lookIs the report wrong? Write yes or no, and one reason.

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Step 2 of 5 · Look

The average day, month by month

Los Angeles 311, 2022. September is marked.

How to read it. Each bar is one month. Its height is the average number of requests per day that month.

Find out

Find September. Does anything look strange about it?
Which month had the strangest day of 2022? Can you see that day here?

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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=spread_la_311_daily_by_month.csv&mark=boxplot

Under across, pick month. Under up, pick requests. This is a box plot: each box shows the spread of one month's days, and the line inside is the middle day. Read September's middle day.
Hover September's lowest point. What day is it?
Which month stretches the farthest from top to bottom? Read its biggest and smallest day.
Click the dotplot shape and hover the lowest dot of the whole year. Then go back to the box plot. Where is that day now? Could you see it on the chart of averages?
Name one number about you that is an average, like a grade. What does it hide?

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Step 4 of 5 · Say it

Your answer, in one sentence

I think the report <is or is not right> 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 right, but it hides something, because September's average looks normal even though it includes one terrible day. I checked by switching to a box plot, and that day showed up as a dot below September that the chart of averages never showed.

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Step 5 of 5 · What you found

Check your numbers

3,620

September's average day, right in the middle

3,913

September's middle day

March: 5,877

the month with the widest spread, top day minus bottom day

The big idea: an average hides the extremesAn average squashes every day into one number, so the strangest days disappear. Nobody lied. A box plot or a dot plot keeps the unusual days where you can see them. When you see an average, ask what it is hiding.

One more thing to think aboutSeptember's middle day is 3,913 and its average is 3,620. The one terrible day pulled one of them down. Which one? Which should a report use?

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After the dive

The charts you used today

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.

the box plot

The middle half of the data sits in the box. It shows the spread, not how many.

What it leaves outThe box hides how many. Five values and five hundred draw the same box, and a gap in the middle of the data does not show at all.

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

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