Do Median age and PISA reading go together across countries?

Across 87 countries the link before adjusting for anything is r = +0.71, on a scale from -1 (opposite) to +1 (in step). A version that uses only each country's rank gives +0.68.

Does the link hold within countries over time, not just across countries?

Comparing each country only with itself across 2000-2022, and setting aside what happened worldwide in each year, the link is +0.081 (57 countries, 411 country-years). Using year-to-year changes, it is —.

Does Median age cause PISA reading?

This analysis cannot answer that. It shows only that the two go together. A link between two country measures, adjusted or not, does not show that either one changes the other. Country figures also say nothing about individual people.

Median age vs. PISA reading

Holds up after adjusting for income

Across 87 countries, Median age and PISA reading rise and fall together at r = +0.71 (on a scale from −1 to +1, where 0 means no link). Comparing only countries with similar income (GDP per person) shrinks it to +0.49 (q = 0.011, allowing for the many pairs tested). Much of the first figure is that both measures rise with national income, but a moderate positive link remains.

Comparing countries across the middle half of pisa reading, 384 to 479 PISA points, median age is 8.07 years higher at the top of that range than the bottom. Comparing only countries with similar national income, the gap is 7.01 years.

Link before adjusting (r)+0.711p = 1.2e-14 · 87 countries

After adjusting for national income+0.486p = 2.1e-6 · likely range [+0.31, +0.63]

p, allowing for regions0.001standard error 0.063 · 5 regions

Check for luck (q)0.011across 12,497 pairs

How to read these.r runs from −1 (one goes up as the other goes down) to +1 (they rise together), and 0 means no link. Adjusting for national income means comparing countries with the same GDP per person, so differences in wealth can’t explain the link. Income is used in percentage terms, since an extra $1,000 means more to a poor country than to a rich one. p is how likely a link this strong would be if there were really none. Below 0.05 means it is unlikely to be chance. Very small values are written in short form: 1.1e-32 means a decimal point followed by 31 zeros and then 11, so very close to zero. The likely range is where the true figure probably lies, and a range that crosses zero means the link could easily be chance. Countries in the same region tend to resemble each other, so the third figure allows for that, which gives a fairer p. The standard error is the typical size of the error in r. Finally, testing this many pairs means some will look strong by luck alone. A q below 0.05 means this link is unlikely to be one of those.

Median age · yearslink with income +0.86 · proxy

PISA reading · PISA pointslink with income +0.77 · high

Both of these rise and fall closely with national income, so a strong link between them before adjusting was close to certain. Read the adjusted figure, not the first one.

“Link with income” is how closely each measure tracks GDP per person across countries, from −1 to +1.

Every country as a dot

best-fit line line after adjusting

Four ways to measure the link

Method	r	p	Countries
Standard (Pearson), not adjusted	+0.711	1.2e-14	87
By rank (Spearman), not adjusted	+0.677	5.9e-13	87
Standard (Pearson), adjusted for national income	+0.486	2.1e-6	87
By rank (Spearman), adjusted for national income	+0.501	9.0e-7	87

The rank version uses only each country’s place in line, from lowest to highest, so a few extreme countries count for less. The adjusted and unadjusted versions use the same 87 countries: those that report both measures and GDP per person. So the gap between them comes from the adjustment, not from a change in which countries are counted. Drop in the size of r after adjusting: +0.225 (a minus sign means it grew).

How solid is it?

Leaving out one country at a time, r ranges from +0.455 (without SAU) to +0.516 (without MKD). The bar above shows that range on a scale from −1 to +1, with the solid line at the full result. The single country that moves r most: SAU (-0.031).

Within each region

Countries are not separate, unrelated cases, and neighbors share a lot. A link that holds inside each region is far more believable than one that shows up only when comparing regions.

	n	r
East Asia & Pacific	15	+0.57
Europe & Central Asia	44	+0.52
Latin America & Caribbean	15	+0.79

By income third

If the link shows up only in the richest third of countries, it is a feature of rich countries, not a general pattern.

	n	r
Poorest third	29	+0.29
Middle third	29	+0.49
Richest third	29	+0.67

By largest religious group

Countries grouped by a label they share, such as main religion, rather than by a number. Groups of fewer than twelve countries are not shown. These groups differ in income, region and history all at once. So a gap between them is a place to start asking questions, not an answer.

	n	r
Christians	55	+0.43
Muslims	18	+0.20

By last european coloniser

	n	r
Spain	14	+0.55
United Kingdom	14	+0.48

Without outliers (countries far outside the middle half of countries on either measure, by more than one and a half times the width of that middle half): r = +0.486 after dropping 0 countries.

Two series, one country

Both measures over the years for one country you pick. The two charts are stacked so they share only the years along the bottom.

Loading…

Over time, within countries

Everything above compares countries with each other at one moment. This section compares each country only with itself over 23years. So a link here can’t come from anything about a country that stays the same, such as its history, geography, institutions or culture.

Within countries over time, there is no link that is clearly more than chance (r = +0.081, comparing each country only with itself over the years, and each year only with the same year elsewhere). Whatever the cross-country picture shows, countries whose median age moved did not tend to see their pisa reading move with it.

Across countries (adjusted)+0.486for comparison

Within countries-0.018each country vs itself

…and vs the same year elsewhere+0.081q = 0.607 · 57 countries

Year-on-year changes—q = —

Read with care.

Almost all of this indicator's variation is between countries, not within them, so the within-country estimate rests on a small part of the data.

411 country-years (one country in one year), 57 countries, 2000–2022, about 7 years each on average. Share of the variation that happens within countries over time: 8% for Median age, 13% for PISA reading. The margins of error allow for each country appearing many times, and for each year’s value being close to the year before.

Year-to-year changes are a cautious test, not a final answer. Looking only at changes from one year to the next removes each country’s own trends. But it also makes measurement mistakes louder, which pulls the results toward zero. A near-zero result here is weak evidence of no link, not proof.

With one measure years behind the other

Same year (lag 0): +0.081. A peak away from zero does not show which comes first. These series change slowly — this year’s value is close to last year’s. That makes the curve smooth and nearly symmetric, whatever produced the data. Even a sharp peak at one lag (a gap of some years) would not prove that one measure changes the other.

This curve peaks at the edge of the window (lag -8), so it is still drifting where the search stops. That pattern usually reflects a slow trend left over in the data, not a link with real timing. The peak value should not be read as the “true” link at the right lag.

The data

	years	PISA points	Years
Japan JPN	49	516	2023, 2022
Italy ITA	47.5	482	2023, 2022
Portugal PRT	46.2	477	2023, 2022
Hong Kong SAR, China HKG	46.2	500	2023, 2022
Greece GRC	45.7	438	2023, 2022
Germany DEU	45.1	480	2023, 2022
Bosnia and Herzegovina BIH	44.9	403	2023, 2018
Spain ESP	44.9	474	2023, 2022
Croatia HRV	44.8	475	2023, 2022
Korea, Rep. KOR	44.5	515	2023, 2022
Bulgaria BGR	44.3	404	2023, 2022
Slovenia SVN	43.9	469	2023, 2022
Serbia SRB	43.9	440	2023, 2022
Hungary HUN	43.3	473	2023, 2022
Austria AUT	43.1	480	2023, 2022

United Nations, Department of Economic and Social Affairs, Population Division: World Population Prospects 2024 and Estimates and Projections of Women of Reproductive Age Who Are Married or in a Union
OECD databases as named on each indicator: PISA, Social Expenditure Database (SOCX), Income Distribution Database, Development Assistance Committee statistics, Family Database, Government at a Glance, Corporate Tax Statistics, employment and environment statistics

Sources

Median age

Median age of the population

United Nations Department of Economic and Social Affairs, Population Division · via Our World in Data · CC BY 3.0 IGO · updated biennial

The age that splits a population in half: half are older and half are younger. The shares aged under 15 and over 65 are already on the site. All three are treated as measuring the same thing. They are readings of the same age pyramid, so they are bound to match, and their agreement is not a finding. No "better" direction is set.

PISA reading

PISA mean reading score

OECD · via Our World in Data · CC BY 4.0 (OECD terms and conditions) · updated annual, except the snapshots noted per indicator

How this was computed

Data used: most recent value in 2010-2025; values come from 2015–2023. To adjust for income, each measure is first predicted from GDP per person (in percentage terms, on a log scale) with a straight-line fit. Only what is left over is then compared. This is called a partial correlation. The line is drawn the standard way (ordinary least squares), which picks the line that comes closest to all the countries at once. The error margins allow for countries in the same region resembling each other, using 5 World Bank regions. The q-value corrects for testing all 12,497 pairs at once, using the Benjamini–Hochberg method. Underlying data last updated DYB 2024. How the country data works · explore Median age and PISA reading

This page shows that two measures go together. It does not show that Median age changes PISA reading, or the reverse. These figures also describe countries, not individual people. See the ecological fallacy.