Updated 12 October 2026 · By Pete Bromfield, former Head of Maths with IB examining experience

IA idea · Statistics & hypothesis testing

Are elite footballers more likely to be born early in the selection year?

AI SLAI HLAA SLAA HL Accessible Also in: Sport, Probability

Research question

Are the birth months of players in [a league or national youth team] uniformly distributed, or are players born soon after the age-group cut-off over-represented?

Make it yours, then agree it with your teacher.

Next step: plan this IA free — the planner opens with this question filled in. For feedback on your own draft, the IA package (€39 once) gives you 5 AI feedback credits and every annotated exemplar in full.

Why it makes a good exploration

The relative age effect — older children in a school year are picked for teams more often — is well documented in youth sport. Testing it for a squad you follow is a clear chi-squared question with real-world implications.

The mathematics you'll need

Chi-squared goodness-of-fit against expected births by month
Using national birth statistics for expected frequencies
Grouping into quarters to keep expected counts above 5
Comparing senior and youth squads

The statistics, step by step

Worked with every number shown, with what examiners look for and the common mistakes: Chi-squared goodness of fit. Then run the same steps on your own data in Analyse my data, or start from the statistics workflow.

Where the data comes from

Collect dates of birth of 200+ players from FBref squad pages or Olympedia; use national birth data for expected frequencies.

FBref — Football league tables, results and basic player stats. Advanced stats (xG, progressive passes) were removed in January 2026 — historical basic data is still there.
Olympedia — Complete Olympic results for every event since 1896.
Office for National Statistics (UK) — UK population, earnings, inflation, births and deaths time series.

Real data for this idea

Primary source: Office for National Statistics (UK): Births in England and Wales. The release tables include births by month of occurrence. Licence: Open Government Licence v3.0: free to reuse with an acknowledgement.
Also useful: US births per day (NCHS and SSA, via FiveThirtyEight): US births by day, 1994–2014. Two CSVs (1994–2003 from NCHS, 2000–2014 from SSA) give year, month, day of month, day of week and births. Licence: CC BY 4.0 (FiveThirtyEight's data repository); cite NCHS and SSA as the original sources.

If you query CDC WONDER for more recent US births yourself, its data-use rules forbid publishing any count of 9 or fewer, so merge small cells before you show them. Links checked October 2026; we link to the data and never copy it. More free data sets.

A possible outline

Explain the relative age effect and the cut-off date.
Collect birth months; justify the population used.
Test against uniform and against real birth-month frequencies.
Compare youth and senior levels.
Reflect on selection and fairness.

Pitfalls that cost marks

Assuming births are uniform across months without checking.
Using a squad whose cut-off date differs.
Too few players for monthly categories.

Showing personal engagement

Use your own club or school team as a mini sample.
Discuss whether you have experienced this effect.
Suggest a policy (e.g., rotating cut-offs) and justify it.

See Criterion C: personal engagement for what examiners look for.

Which course is it for?

Course	Fit	Maths to lean on
AA SL	Good fit	Chi-squared goodness-of-fit against expected births by month; Using national birth statistics for expected frequencies
AA HL	Fits, but add an HL technique	Chi-squared goodness-of-fit against expected births by month; Using national birth statistics for expected frequencies
AI SL	Good fit	Chi-squared goodness-of-fit against expected births by month; Using national birth statistics for expected frequencies
AI HL	Fits, but add an HL technique	Chi-squared goodness-of-fit against expected births by month; Using national birth statistics for expected frequencies

Level: Accessible. A good first extended piece of maths, with room to go deeper. See how the IA differs between AA and AI, SL and HL.

How this idea reaches the top bands

Reflection (D)

For this idea, start with the weak points: assuming births are uniform across months without checking; and using a squad whose cut-off date differs. Say how each one affects your answer.

Use of mathematics (E)

SL: chi-squared goodness-of-fit against expected births by month; using national birth statistics for expected frequencies; grouping into quarters to keep expected counts above 5; comparing senior and youth squads, each explained in your own words and carried out correctly.

How statistics explorations reach the top bands in general: Statistics & hypothesis testing IA ideas. Guides: A · B · C · D · E · self-grade sheet.

Taking it further

Compare sports with different cut-off dates, or test whether the effect is stronger for goalkeepers or strikers.

Extending it for HL

Add an HL technique the question genuinely needs: what an HL statistics exploration adds.

See a complete IA, marked

Should I lease or buy my first car? (AI SL) is a complete statistics exploration, marked on every criterion.

Frequently asked questions

Which IB Maths course is this IA idea for?

It works for AI SL, AI HL, AA SL and AA HL. We rate it Accessible: a good first extended piece of maths, with room to go deeper.

What mathematics does this IA use?

Chi-squared goodness-of-fit against expected births by month, using national birth statistics for expected frequencies, grouping into quarters to keep expected counts above 5 and comparing senior and youth squads.

Where can I get data for this IA?

Collect dates of birth of 200+ players from FBref squad pages or Olympedia; use national birth data for expected frequencies. Checked sources and tools: Office for National Statistics (UK), US births per day (NCHS and SSA, via FiveThirtyEight), FBref and Olympedia.

Turn this idea into your IA

The planner opens with “Are elite footballers more likely to be born early in the selection year?” filled in: the question, the maths and the data sources.

Plan this IA free

Your plan is kept in this browser. A free account keeps it with your drafts on any device: Join free · Sign in

Also: free IA self-check · A–E checklist (PDF) · have a draft? AI feedback or a review with Pete (€100)

Writing it up: how to write a Maths IA, step by step · structure and format · mistakes that cost marks · IA statistics, step by step