The picked time window

By the end of this lesson you can see how choosing the start and end dates can change the story.

Why it matters

"Sales doubled!" "Crime is falling!" Each claim is about a period of time. Pick the right start and end, and almost any series can be made to rise or fall. Checking the window protects you from stories that are true but misleading.

The idea in plain words

Imagine a photo of a football match taken the moment one team scores. It looks like they are winning. But the full match ended 3 to 1 to the other team. The photo is real. It just shows one moment, picked from a longer story.

A time chart works the same way. Show only a few years, and you see only that part of the story.

Time window: the start and end dates a chart or a claim covers, such as "2009 to 2013".

Here is Japan's CO₂, shown for 2009 to 2013 only.

A short window, and an axis that starts at 1,100. It looks like a steep climb.

See it done

Data: country-indicators.csv (real), Japan's rows, column F co2_million_tonnes.

Claim to check: "Japan's CO₂ is shooting up."

1. Find the window. The chart covers 2009 to 2013 only: 5 years out of 35 in the data.
2. Check the axis. It starts at 1,100, not 0, so the rise looks steeper than it is (lesson 5.1).
3. Read the window's numbers. 2009: 1,160.3 million tonnes. 2013: 1,311.9 million tonnes. A rise of 1,311.9 − 1,160.3 = 151.6.
4. Look outside the window. Draw (or view) all the years, 1990 to 2024, with the axis from 0.

The whole story: 2013 was the peak. Since then it has mostly fallen, down to 961.9 in 2024, its lowest value in the period.

5. Read the full story. 1990: 1,154.9. Peak in 2013: 1,311.9. 2024: 961.9, the lowest value of all 35 years.
6. Judge the claim. "Shooting up" was true for 2009 to 2013 only. Over the whole period, and especially since 2013, Japan's CO₂ has fallen. The claim picked a window that hides the rest.

To make the full chart yourself: in the country tab, Japan's rows are together (the data is sorted by country, then year). Copy its year and co2_million_tonnes cells into a small table under typed headers, as in the Module 4 assignment, and insert a Line chart.

Your turn, with help

Someone says, "Since 2013, Japan's CO₂ has fallen by more than a quarter." Check it.

Hint: work out the fall (2013 value minus 2024 value), then compare it with a quarter of the 2013 value (2013 value ÷ 4).

Show the answer

Fall: 1,311.9 − 961.9 = 350.0. A quarter of 1,311.9 is 328.0 (rounded). 350.0 is more than 328.0, so the claim is true: it fell by more than a quarter (about 27 percent).

Your turn, alone

1. Using the full Japan chart, name a window of a few years in which the line mostly falls.
2. A headline says "Japan's CO₂ fell by 350 million tonnes since 2013." Is the number right? Is anything missing?
3. Amara's busiest day was Friday (12 sales). If she only showed you Wednesday to Friday (8, 11, 12 sales), what story would that tell? Is it the whole story?
4. Write two questions you should always ask about a time window.

Check your answers

1. For example, 2013 to 2024, or 2018 to 2024.
2. Yes: 1,311.9 − 961.9 = 350.0. Missing: that 2013 was the highest year, so starting there makes the fall look as big as possible. Starting in 1990 (1,154.9), the fall to 2024 is 193.0.
3. "Sales rose every day", from 8 to 11 to 12. It leaves out that Monday had 10 and Saturday fell back to 10.
4. For example: why does it start and end there? What happened before and after?

Watch out

• Starting at a peak or a dip. A start at the highest point makes any fall look big; a start at the lowest makes any rise look big.
• Very short windows. A few points can show a "trend" that is really just normal ups and downs.
• Hiding the window. If a claim does not say which years it covers, ask.

Say it back

1. What is a time window?
2. What was Japan's CO₂ in its peak year, and which year was it?
3. What two questions should you ask about any time window?

Show the answers

1. The start and end dates a chart or claim covers.
2. 1,311.9 million tonnes, in 2013.
3. Why does it start and end there? What happened before and after?

What is next

Lucía's two classes take exactly the same time to get to school, on average: 20 minutes. So their mornings must be alike, right? Not even close. Next lesson.

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Data in this lesson: Global Carbon Budget (2025); UN World Population Prospects (2024), processed by Our World in Data. Licensed under CC BY 4.0 (UN data: CC BY 3.0 IGO). Extract and continent labels by DataMedics.