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.

2 · intermediate · cartesian · level 2 · ask who counted · taught in dive 3.2 · dive 2.6

GAISE II · Level B p.45

Three worked examples

Which month was the most unpredictable?

Before you lookWrite down the month you think varied most from day to day. Not the busiest month — the least consistent one.

civicca_schools_poverty_and_tests.csv · 9,480 rows · 6 columns · 6 of the file’s 12 columns are on this page; the download has all of them

how many students met the state standard in mathematics, at every one of California's 9,480 schools, grouped by elementary, middle and high

What to look atFour boxes for nine and a half thousand schools — and read them left to right, because that is the order a student goes through them: the median falls 32 to 25 to 15 as the grades climb. The box holds the middle half; it says nothing about HOW MANY schools are in it, and nothing about the one your own school sits in. The fourth box is 44 schools whose grade span fits none of the three.

This example draws itself with a small script. With scripting off you can still read every word on this page, and the file itself is at data/ca_schools_poverty_and_tests.csv.

earth and spacehist_quake_magnitudes.csv · 2,508 rows · 1 columns · showing 400 of 2,508 rows on this page; the file has all of them

the magnitude of all 2,508 southern California earthquakes

What to look atOne box holding 2,508 earthquakes. It shows how spread out they are and tells you nothing about how many — that is the trade.

This example draws itself with a small script. With scripting off you can still read every word on this page, and the file itself is at data/hist_quake_magnitudes.csv.

teen healthteen_experience_spread.csv · 290 rows · 4 columns

ten things about teenagers, measured for every group

What to look atTen boxes, twenty-nine groups each, all in per cent of teens. The tallest box is whether you play a sport — its middle half alone runs 53 to 62. The flattest by far is two hours of screens, a box from 92 to 93 — the one thing nearly every group does alike. Read the boxes as a ranking of what teenagers actually differ on.

This example draws itself with a small script. With scripting off you can still read every word on this page, and the file itself is at data/teen_experience_spread.csv.

the pool

Now build one yourself.

The pool opens with this file and this shape already drawn, so your first move is to change something rather than to start from nothing. Bring your own CSV if you have one. Nothing is uploaded anywhere — the chart is drawn in your browser, your work saves there, and it downloads as a picture or as a project file you can open again.

Open the pool → the box plot · ca_schools_poverty_and_tests.csv