What Is a Diagram For?
All six diagram typesPart I · Framing · Chapter 1 of 12 · the opening chapter
What does a diagram show that a paragraph doesn’t? A diagram puts every cause of a quantity at that quantity, shows a circle that a paragraph’s order hides, and, with dashed lines, shows which effects its author judges less supported.
Print this chapter, with a sheet for each exercise and the answers at the back, from its PDF.
Empty shops on the high street
Section titled “Empty shops on the high street”The spring newsletter of a market town’s traders opened with this paragraph:
Since the council raised parking charges, fewer people come into town. The council’s own footfall counter shows visitors down by a fifth over two years, and our own figures show takings per shop falling with them, month by month. Seven shops have closed, and every owner gave falling takings as the reason. Every empty shop gives people one less reason to come, so fewer still come. Some blame online shopping, but people shopped online before the charges went up.
What the notation expresses
Section titled “What the notation expresses”Variables
Section titled “Variables”A variable is a quantity that can rise or fall, named as a noun phrase: visitors to the high street, takings per shop. An action is not a variable: shoppers stay away names something that happens, and visitors to the high street names what rises or falls when it does. The test for a name is in chapter 4.
Effects
Section titled “Effects”An effect is a line with an arrowhead from one variable to another: a change in the first changes the second. Takings per shop → empty shops: as takings fall, shops close.
Every figure in the chapter is the start of a causal loop diagram, whose effects gain a polarity, + or −, for which way the change runs: by the upgrade in chapter 3, where a direction has been measured, and on every effect in chapter 4.
Evidence coding
Section titled “Evidence coding”Evidence coding shows the author’s judgment of how well supported each effect is as a causal mechanism: in a diagram with a dashed effect, a solid effect is one the author judges more supported, and a dashed one less supported, a hypothesis. A dashed line means nothing else in this book.
Support comes in kinds, some stronger than others: a comparison of places where the first variable changed with places where it didn’t; a count of the second changing alongside the first; an explanation of how the first changes the second; testimony; a hunch. Where the line between solid and dashed falls is the author’s decision: one author draws solid only what a comparison supports, another anything better than a hunch, and a reader can question either.
The traders draw theirs at a count. They counted takings falling with visitors, month by month, and draw visitors → takings solid; the owners’ reasons for closing are testimony, and takings → empty shops is dashed, though a trader who took the owners at their word could draw it solid.
A variable counted alone supports no effect. The footfall counter measures visitors to the high street, down by a fifth, but nobody counted visitors alongside a change in the charges, in online shopping or in empty shops, so the traders dash those three effects too.
A diagram with no dashed effect carries no coding: its solid effects are its author’s claims, their support unrecorded.
The legend
Section titled “The legend”The legend is a box listing the codings a diagram uses; any diagram with a dashed effect must have one. Its evidence entry shows a solid line for more supported and a dashed line for less supported. It also lists any viewpoint tags, chapter 5’s small boxes naming who stresses an effect.
The drawing and the paragraph
Section titled “The drawing and the paragraph”The paragraph runs from the charges to the shops; the drawing shows four things its order hides:
- A circle. Fewer still come leads back to the first variable: visitors → takings → empty shops → visitors is a closed path, in chapter 4 a loop.
- Two causes at one variable. Parking charges and online shopping both end at visitors to the high street. The traders blame one and dismiss the other, on the same evidence: none.
- Causes with nothing into them. Nothing drawn changes the charges or online shopping.
- Where the traders draw the line. Of the circle’s three effects, only visitors → takings is solid.
In the paragraph, what bears on visitors is spread over three sentences; in the drawing, it is the three lines ending at visitors. A diagram puts at one place what is used together, and shows what a paragraph leaves to be worked out (Larkin and Simon, 1987).
Reading someone else’s diagram
Section titled “Reading someone else’s diagram”Of each effect, solid or dashed, ask what changes; whether a change in the first changes the second, or only comes before it; and how anyone knows. Of the coding, ask where the author drew the line between solid and dashed, and whether you would draw it there: a solid effect is its author’s judgment, not proof that one change causes the other. Of the whole, ask what is missing: a cause nobody drew, or an effect that would close another circle.
The lab’s checks test structure, never whether an effect holds: a check counting dashed effects passes a diagram with the wrong ones dashed.
The bad diagram
Section titled “The bad diagram”What’s wrong with this diagram, from a youth club’s bid to the council for another year’s funding?
Every effect is solid, and three rest on counts: the club’s sign-in sheets count attendance by evening, the council publishes its youth grant each year beside complaints to the police in the club’s area, and the club’s accounts show how many evenings the grant pays for. That attending keeps young people off the streets at night, that fewer on the streets means fewer complaints, and that more volunteers would open more evenings are the club’s beliefs. The club draws no line between counts and beliefs, so a reader can’t tell the sign-in sheets from a belief about the streets at night.
The corrected diagram
Section titled “The corrected diagram”Labels unchanged. 3 effects dashed as less supported: young people attending → young people on the streets at night, young people on the streets at night → complaints to the police and volunteers → evenings open a week; legend switched on.
A count of young people on the streets near the club, on nights it opens and nights it doesn’t, would give the club grounds to draw young people attending → young people on the streets at night solid.
The central diagram types
Section titled “The central diagram types”| Type | Adds to variables and effects | Chapters |
|---|---|---|
| system context map | A named boundary, the parties outside it in ovals, and named exchanges crossing it, in place of variables and effects | 2 |
| influence map | Influence claims, effects with no polarity; weight in dots; one central outcome; clusters | 3 |
| causal loop diagram | A polarity on every effect; loops; delays; boundaries and external drivers | 4–5 |
| stock & flow diagram | Stocks, the flows that fill and drain them, clouds, and signals carrying readings to flows and to other variables | 6–7 |
| feedback control diagram | A role written above each variable that plays one, over the causal loop diagram’s marks, its effects called signals; where the controlled variable accumulates, a stock and its flows | 8–9 |
| system archetype | A declared archetype and roles, over the causal loop diagram’s marks | 10–11 |
Which type to draw for which question is in chapter 12. You draw each exercise as one type in the lab, with only the marks you need for it.
What stays informal
Section titled “What stays informal”One claim, or several that don’t meet, go in a sentence: the charges keep shoppers away. Causes with no effects between them go in a list. The chapter’s figures show no direction, amount or timing; each is drawn from a later chapter on. Each type’s case for a sentence instead is in chapter 12.
Notation moves
Section titled “Notation moves”| Notation move | When | What it shows | Example |
|---|---|---|---|
| Name a variable | What rises or falls | takings per shop, not shops lose money | |
| Draw an effect | A change in one quantity changes another | What changes what | takings per shop → empty shops |
| Dash a hypothesis | You judge the effect less supported | Where you drew the line between more and less supported | empty shops → visitors to the high street |
| Add a legend | An effect is dashed | The codings in use | Evidence: more supported, less supported |
| Ask how they know | You read someone else’s diagram | What supports each effect, and where the author drew the line | young people attending → young people on the streets at night |
Common misreading
Section titled “Common misreading”Read one way, a solid effect is proven everywhere. Its support was gathered somewhere, under some conditions: induced traffic, solid on chapter 5’s road, was found across American cities (Duranton and Turner, 2011), and a town of another shape may differ.
Read the other way, a dashed effect is false, or minor. It is less supported, not less important: the high street’s dashed empty shops → visitors closes the circle.
The strongest objection
Section titled “The strongest objection”Evidence comes in degrees, and the coding has two: an effect counted in one town and one counted in twenty are drawn with the same solid line. The IPCC’s authors grade evidence as limited, medium or robust, agreement as low, medium or high, and an outcome’s likelihood in seven steps, from virtually certain to exceptionally unlikely (Mastrandrea and others, 2010).
With the coding, each effect carries one answer, which side of its author’s line it falls on, readable at a glance. Where the line falls, and the degrees on either side of it, go in the sources and the prose beside the drawing, where the IPCC’s authors put theirs: a traceable account of each finding’s evidence, in the text.
Check your understanding
Section titled “Check your understanding”Foresight’s obesity system engine, redrawn
Section titled “Foresight’s obesity system engine, redrawn”Redrawn from Philippe Vandenbroeck, Jo Goossens and Marshall Clemens, Tackling Obesities: Future Choices – Obesity System Atlas (Foresight, Government Office for Science, 2007), Map 19, System Engine + 4 Key Variables. Contains public sector information licensed under the Open Government Licence v3.0.
Map 19’s ten variables, of about 110 in the whole map, and the 22 effects among them; its solid arrows, marking a positive influence, and its dotted lines ending in a square, marking a negative one, all drawn solid with an arrowhead and no polarity; its colors and the thick bands marking the engine left out; its names lowercased, two shortened.
Exercises
Section titled “Exercises”Passing the checks means meeting the brief, not being right.
DF 01Greta's morning drawing
A paragraph drawn
Greta draws for twenty minutes before work each morning and keeps a log. After three months she wrote in it: “I time one sketch every morning, and it now takes me five minutes, down from nine. My times fell fastest in the weeks I drew longest, and not at all in the fortnight I was ill and barely drew. Beside each sketch I mark from one to five how much I enjoyed the session, and the mornings I sketched fastest are the ones I marked highest. On those mornings I keep going past twenty minutes, and the log shows it. I started an online course in January, and I think its lessons have made me faster too, but I've kept no count of the lessons beside my times.”
The brief
- Draw four variables. checked by the lab
- Draw an effect wherever Greta writes that one quantity changes another. checked by the lab
- Code the one effect with the weakest support as a hypothesis, so it is drawn dashed. checked by the lab
- Each name is a quantity that can rise or fall: minutes drawn each morning, not draws every morning. you check this
- The effects close a circle. you check this
Your palette
variable effect evidence coding
To draw in the lab, use a tablet or a larger screen. Everything else reads normally here.
Press an empty spot to add a variable and type its name. Drag a variable to move it. Select a variable, an effect or a loop label to change it. Double-click or double-tap a variable to edit its words in place, or select it and use Rename or the fields under the canvas.
Nothing selected. Select a variable or an effect on the canvas, and what you can change shows here.
The tools
- Variables V
- add a variable (press an empty spot), move one (drag it), rename one (double-click it, or Rename under the canvas), or select anything.
- Effects E
- draw an effect by dragging from one variable to another.
Keys
- G legend on or off
- R reverse the selected effect
- Delete delete what's selected
- Ctrl+Z undo
- Ctrl+Shift+Z redo
- Esc clear the selection
- Enter finish the words you're writing
- Esc cancel them
Keys act on the lab you last used, or on the open lab view, and never while you type.
0 of 5 checks passed; 5 pending.
- Draw four variables. (Not started)
- Draw an effect wherever Greta writes that one quantity changes another. (Not started)
- Code the one effect with the weakest support as a hypothesis, so it is drawn dashed. (Not started)
Always checked
- The legend is on when effects are coded or tagged (Not started)
- No variable's name breaks the more test's three rules (Not started)
Written feedback
One defensible answer
It opens here once you've drawn at least one variable in the lab above. Your attempt saves as you draw.
DF 02Esther's grandchildren's visits
Two readings of one calendar
Esther marks each visit from her grandchildren on her kitchen calendar. “Until my son's family moved two hours' drive away, they came every month. Since the move they've come four times a year, and the calendar shows the drop from the first month after it. Last spring the eldest took a Saturday job at a bakery, and I think that keeps them all away now, though I haven't compared the calendar with her shifts. And I think the less often the children come, the less they want to come, and the less they want to, the less often they come. I haven't asked them.” Her son agrees that the drive cut the visits, and says it alone explains them.
The brief
- Draw four variables. checked by the lab
- Draw an effect wherever Esther or her son says one quantity changes another. checked by the lab
- Code the three effects with the weakest support as hypotheses, so they are drawn dashed. checked by the lab
- Each name is a quantity that can rise or fall: visits a year, not the grandchildren visit. you check this
- Esther and her son disagree only about dashed effects. you check this
Your palette
variable effect evidence coding
To draw in the lab, use a tablet or a larger screen. Everything else reads normally here.
Press an empty spot to add a variable and type its name. Drag a variable to move it. Select a variable, an effect or a loop label to change it. Double-click or double-tap a variable to edit its words in place, or select it and use Rename or the fields under the canvas.
Nothing selected. Select a variable or an effect on the canvas, and what you can change shows here.
The tools
- Variables V
- add a variable (press an empty spot), move one (drag it), rename one (double-click it, or Rename under the canvas), or select anything.
- Effects E
- draw an effect by dragging from one variable to another.
Keys
- G legend on or off
- R reverse the selected effect
- Delete delete what's selected
- Ctrl+Z undo
- Ctrl+Shift+Z redo
- Esc clear the selection
- Enter finish the words you're writing
- Esc cancel them
Keys act on the lab you last used, or on the open lab view, and never while you type.
0 of 5 checks passed; 5 pending.
- Draw four variables. (Not started)
- Draw an effect wherever Esther or her son says one quantity changes another. (Not started)
- Code the three effects with the weakest support as hypotheses, so they are drawn dashed. (Not started)
Always checked
- The legend is on when effects are coded or tagged (Not started)
- No variable's name breaks the more test's three rules (Not started)
Written feedback
One defensible answer
It opens here once you've drawn at least one variable in the lab above. Your attempt saves as you draw.
DF 03Hedgehogs in Britain's countryside
Sorting a report's evidence
A county wildlife trust's newsletter sums up the report The State of Britain's Hedgehogs 2022 for its members. “Surveys show rural hedgehogs down by between a third and three-quarters in twenty years. Fewer are found dead on roads too, perhaps three-quarters fewer since 1990, though traffic has grown: with fewer hedgehogs about, fewer are killed. Whether road deaths are behind the decline, the report's authors call unclear. Badgers eat hedgehogs and compete with them for earthworms and beetle larvae, and the authors call it likely that badgers reduce hedgehog numbers where there are many of them. Studies show that wide, grassy field margins hold more earthworms and ground beetles; whether hedgehogs are fewer where their prey is scarcer is among the questions the authors list as still being studied. Researchers found that on arable land hedgehogs seek out village gardens to feed and are densest there, and a small hole at the foot of a fence opens one garden to the next.”
The brief
- Draw at least five variables. checked by the lab
- Draw an effect wherever the report's authors find or suspect that one quantity changes another. checked by the lab
- Code at least two effects as hypotheses, so they are drawn dashed. checked by the lab
- Each name is a quantity that can rise or fall: hedgehogs killed on roads each year, not roadkill. you check this
- An effect is solid where you judge its support strong enough; when the authors call an effect likely, that is testimony, weaker than a count. you check this
Your palette
variable effect evidence coding
To draw in the lab, use a tablet or a larger screen. Everything else reads normally here.
Press an empty spot to add a variable and type its name. Drag a variable to move it. Select a variable, an effect or a loop label to change it. Double-click or double-tap a variable to edit its words in place, or select it and use Rename or the fields under the canvas.
Nothing selected. Select a variable or an effect on the canvas, and what you can change shows here.
The tools
- Variables V
- add a variable (press an empty spot), move one (drag it), rename one (double-click it, or Rename under the canvas), or select anything.
- Effects E
- draw an effect by dragging from one variable to another.
Keys
- G legend on or off
- R reverse the selected effect
- Delete delete what's selected
- Ctrl+Z undo
- Ctrl+Shift+Z redo
- Esc clear the selection
- Enter finish the words you're writing
- Esc cancel them
Keys act on the lab you last used, or on the open lab view, and never while you type.
0 of 5 checks passed; 5 pending.
- Draw at least five variables. (Not started)
- Draw an effect wherever the report's authors find or suspect that one quantity changes another. (Not started)
- Code at least two effects as hypotheses, so they are drawn dashed. (Not started)
Always checked
- The legend is on when effects are coded or tagged (Not started)
- No variable's name breaks the more test's three rules (Not started)
Written feedback
One defensible answer
It opens here once you've drawn at least one variable in the lab above. Your attempt saves as you draw.
DF 04Objects in orbit
A circle closed by a model
Notes from ESA's Space Environment Report 2026 and NASA's Orbital Debris Quarterly News. Launches add satellites to orbit, and 2025 had the most launches yet counted. Satellites leave orbit at the end of their life, brought down or left to fall: 1,200 intact satellites and rocket stages re-entered the atmosphere in 2025. On 10 February 2009 the working satellite Iridium 33 and the disused Cosmos 2251 collided at more than 11 km a second; both broke apart, and by the end of March 823 of their larger fragments had been catalogued. More fragments come from satellites and rocket stages that break apart without a collision, which ESA counts as the largest source of fragments today. Accidental collisions are few: Iridium 33 and Cosmos 2251 was the fourth known, and the first between two intact spacecraft. That more satellites and fragments in orbit bring more collisions, enough to set off a cascade of them over the coming centuries, comes from models: Donald Kessler and Burton Cour-Palais's in 1978, and ESA's now.
The brief
- Draw at least five variables, with satellites in orbit and fragments in orbit apart. checked by the lab
- Draw an effect wherever one quantity in the notes changes another. checked by the lab
- Code the two effects that come from models, not counts, as hypotheses, so they are drawn dashed. checked by the lab
- Each name is a quantity that can rise or fall: collisions each year, not a collision. you check this
- The two dashed effects, from satellites and fragments in orbit to collisions, come from models, not counts, and close the circles. you check this
Your palette
variable effect evidence coding
To draw in the lab, use a tablet or a larger screen. Everything else reads normally here.
Press an empty spot to add a variable and type its name. Drag a variable to move it. Select a variable, an effect or a loop label to change it. Double-click or double-tap a variable to edit its words in place, or select it and use Rename or the fields under the canvas.
Nothing selected. Select a variable or an effect on the canvas, and what you can change shows here.
The tools
- Variables V
- add a variable (press an empty spot), move one (drag it), rename one (double-click it, or Rename under the canvas), or select anything.
- Effects E
- draw an effect by dragging from one variable to another.
Keys
- G legend on or off
- R reverse the selected effect
- Delete delete what's selected
- Ctrl+Z undo
- Ctrl+Shift+Z redo
- Esc clear the selection
- Enter finish the words you're writing
- Esc cancel them
Keys act on the lab you last used, or on the open lab view, and never while you type.
0 of 5 checks passed; 5 pending.
- Draw at least five variables, with satellites in orbit and fragments in orbit apart. (Not started)
- Draw an effect wherever one quantity in the notes changes another. (Not started)
- Code the two effects that come from models, not counts, as hypotheses, so they are drawn dashed. (Not started)
Always checked
- The legend is on when effects are coded or tagged (Not started)
- No variable's name breaks the more test's three rules (Not started)
Written feedback
One defensible answer
It opens here once you've drawn at least one variable in the lab above. Your attempt saves as you draw.
CS 01Kelsham Primary School: the head teacher's letter
Counts and beliefs in one letter
In October 2026 Ruth Okafor, head teacher of Kelsham Primary School, wrote to parents. “We have 88 pupils on roll this October, against 126 in October 2020. This September 9 children started with us and 16 left, Year 6 among them. The county council's records show each September's starters falling with the births in our villages four years before. Each October's count sets our funding, £4,800 a pupil: when the roll fell to 108 in 2023, it paid for four teachers, not five, and Years 3 and 4 became one class. With 88 pupils in four classes, our classes are small. Some of you tell us that mixed-age classes are why families have moved to Brayford Primary School, and others that small classes are why you stay. Nobody has counted either.”
The brief
- Draw at least six variables, with the children starting each September and the pupils leaving each year apart. checked by the lab
- Draw an effect wherever Ruth Okafor writes that one quantity changes another. checked by the lab
- Code the two effects with the weakest support, the reasons parents give, as hypotheses, so they are drawn dashed. checked by the lab
- Each name is a quantity that can rise or fall: pupils leaving each year, not families move. you check this
- An effect is solid where you judge a count by the school or the county council supports it. you check this
Your palette
variable effect evidence coding
To draw in the lab, use a tablet or a larger screen. Everything else reads normally here.
Press an empty spot to add a variable and type its name. Drag a variable to move it. Select a variable, an effect or a loop label to change it. Double-click or double-tap a variable to edit its words in place, or select it and use Rename or the fields under the canvas.
Nothing selected. Select a variable or an effect on the canvas, and what you can change shows here.
The tools
- Variables V
- add a variable (press an empty spot), move one (drag it), rename one (double-click it, or Rename under the canvas), or select anything.
- Effects E
- draw an effect by dragging from one variable to another.
Keys
- G legend on or off
- R reverse the selected effect
- Delete delete what's selected
- Ctrl+Z undo
- Ctrl+Shift+Z redo
- Esc clear the selection
- Enter finish the words you're writing
- Esc cancel them
Keys act on the lab you last used, or on the open lab view, and never while you type.
0 of 5 checks passed; 5 pending.
- Draw at least six variables, with the children starting each September and the pupils leaving each year apart. (Not started)
- Draw an effect wherever Ruth Okafor writes that one quantity changes another. (Not started)
- Code the two effects with the weakest support, the reasons parents give, as hypotheses, so they are drawn dashed. (Not started)
Always checked
- The legend is on when effects are coded or tagged (Not started)
- No variable's name breaks the more test's three rules (Not started)
Written feedback
One defensible answer
It opens here once you've drawn at least one variable in the lab above. Your attempt saves as you draw.
Sources
Section titled “Sources”- Jill H. Larkin and Herbert A. Simon, “Why a Diagram Is (Sometimes) Worth Ten Thousand Words”, Cognitive Science 11, no. 1 (1987): 65–99, on diagrams that hold at one location what an inference uses, and make explicit what sentences leave implicit.
- Michael D. Mastrandrea and others, Guidance Note for Lead Authors of the IPCC Fifth Assessment Report on Consistent Treatment of Uncertainties (IPCC, 2010), on grading evidence, agreement and likelihood in words, with a traceable account in the text.
- Gilles Duranton and Matthew A. Turner, “The Fundamental Law of Road Congestion: Evidence from US Cities”, American Economic Review 101, no. 6 (2011), measuring how driving rises with new lanes.
- David Wembridge, Grace Johnson, Nida Al-Fulaij and Steve Langton, The State of Britain’s Hedgehogs 2022 (People’s Trust for Endangered Species and British Hedgehog Preservation Society, 2022), for DF 03.
- ESA Space Debris Office, ESA’s Annual Space Environment Report, issue 10.0 (2026), for DF 04.
- NASA Orbital Debris Program Office, “Satellite Collision Leaves Significant Debris Clouds”, Orbital Debris Quarterly News 13, no. 2 (2009), for DF 04.
- Donald J. Kessler and Burton G. Cour-Palais, “Collision Frequency of Artificial Satellites: The Creation of a Debris Belt”, Journal of Geophysical Research 83, no. A6 (1978), for DF 04.