Data story · 11+ · Essex (CSSE) and Buckinghamshire
The Essex 11+ birthday adjustment rose from 8.8 to 14.0 points in a year for the youngest children
CSSE adds points for every day a child is younger than the oldest in the year group, and the amount per day is set afresh from each year's results. In Buckinghamshire, where scores are also age-adjusted, qualifying rates by birth month stayed within 5.5 percentage points of each other in every year.
The 11+ birthday adjustment, and qualifying by birth month
Left: CSSE standardised points added for being younger, same raw marks, by entry year. Right: share of Buckinghamshire children scoring 121+ by birth month, oldest (September) to youngest (August).
- Latest entry year in each panel
- Earlier entry years, named at the line ends
| Area | Entry | Comparison | English age coefficient | Maths age coefficient | Points per day | Days younger | Points added |
|---|---|---|---|---|---|---|---|
| CSSE (Essex) | 2024 entry | Born 1 Sep 2012 vs 31 Aug 2013 | 0.012305 | 0.0061618 | 0.0277 | 364 | 10.08 |
| CSSE (Essex) | 2025 entry | Born 1 Sep 2013 vs 31 Aug 2014 | 0.01620738 | 0 | 0.024311 | 364 | 8.85 |
| CSSE (Essex) | 2026 entry | Born 1 Sep 2014 vs 31 Aug 2015 | 0.01661702 | 0.00894304 | 0.03834 | 364 | 13.96 |
| Area | Entry | Birth month | Children tested | Scoring 121+ | Share |
|---|---|---|---|---|---|
| Buckinghamshire | 2023 entry | Sep | 883 | 321 | 36.4% |
| Buckinghamshire | 2023 entry | Oct | 892 | 342 | 38.3% |
| Buckinghamshire | 2023 entry | Nov | 862 | 331 | 38.4% |
| Buckinghamshire | 2023 entry | Dec | 822 | 304 | 37.0% |
| Buckinghamshire | 2023 entry | Jan | 853 | 323 | 37.9% |
| Buckinghamshire | 2023 entry | Feb | 784 | 303 | 38.6% |
| Buckinghamshire | 2023 entry | Mar | 872 | 292 | 33.5% |
| Buckinghamshire | 2023 entry | Apr | 762 | 266 | 34.9% |
| Buckinghamshire | 2023 entry | May | 807 | 291 | 36.1% |
| Buckinghamshire | 2023 entry | Jun | 737 | 287 | 38.9% |
| Buckinghamshire | 2023 entry | Jul | 742 | 253 | 34.1% |
| Buckinghamshire | 2023 entry | Aug | 737 | 272 | 36.9% |
| Buckinghamshire | 2024 entry | Sep | 888 | 321 | 36.1% |
| Buckinghamshire | 2024 entry | Oct | 962 | 381 | 39.6% |
| Buckinghamshire | 2024 entry | Nov | 870 | 335 | 38.5% |
| Buckinghamshire | 2024 entry | Dec | 806 | 314 | 39.0% |
| Buckinghamshire | 2024 entry | Jan | 839 | 326 | 38.9% |
| Buckinghamshire | 2024 entry | Feb | 713 | 259 | 36.3% |
| Buckinghamshire | 2024 entry | Mar | 789 | 303 | 38.4% |
| Buckinghamshire | 2024 entry | Apr | 710 | 260 | 36.6% |
| Buckinghamshire | 2024 entry | May | 767 | 286 | 37.3% |
| Buckinghamshire | 2024 entry | Jun | 741 | 281 | 37.9% |
| Buckinghamshire | 2024 entry | Jul | 821 | 304 | 37.0% |
| Buckinghamshire | 2024 entry | Aug | 743 | 287 | 38.6% |
| Buckinghamshire | 2025 entry | Sep | 665 | 273 | 41.1% |
| Buckinghamshire | 2025 entry | Oct | 735 | 306 | 41.6% |
| Buckinghamshire | 2025 entry | Nov | 658 | 268 | 40.7% |
| Buckinghamshire | 2025 entry | Dec | 641 | 255 | 39.8% |
| Buckinghamshire | 2025 entry | Jan | 680 | 275 | 40.4% |
| Buckinghamshire | 2025 entry | Feb | 583 | 232 | 39.8% |
| Buckinghamshire | 2025 entry | Mar | 655 | 253 | 38.6% |
| Buckinghamshire | 2025 entry | Apr | 583 | 230 | 39.5% |
| Buckinghamshire | 2025 entry | May | 654 | 238 | 36.4% |
| Buckinghamshire | 2025 entry | Jun | 612 | 231 | 37.7% |
| Buckinghamshire | 2025 entry | Jul | 609 | 227 | 37.3% |
| Buckinghamshire | 2025 entry | Aug | 621 | 244 | 39.3% |
11+; CSSE 2024, 2025, 2026 entry (published coefficients, formula arithmetic) and Buckinghamshire 2023, 2024, 2025 entry (9,753, 9,649, 7,696 children, published score files); points and share scoring 121+; data versions csse-standardisation 2026-09-26.1, bucks-scores 2026-09-26.1
What the data shows
How the adjustment works
CSSE, which runs the 11+ for Essex's selective schools, turns each child's English and maths marks into a standardised total and then adds a fixed amount for every day the child is younger than someone born on 1 September. For 2026 entry that was 0.0383 points a day, so a child born on 31 August gained 13.96 points over a child born 364 days earlier with exactly the same marks. Nothing needs to be claimed: every score already includes it.
The amount changes every year
The per-day amount comes from age coefficients CSSE estimates from each year's own results. The full-year allowance was 10.08 points for 2024 entry, 8.85 points for 2025 entry, 13.96 points for 2026 entry. For 2025 entry the maths coefficient was 0, so only the English paper was adjusted. Around the middle of the 2026-entry scores, 14.0 points is the gap between the 50th and about the 64th percentile.
Outcomes by birth month in Buckinghamshire
Buckinghamshire publishes every child's score by birth month. After its own age standardisation, the share scoring 121 or more ranged from 33.5% to 38.9% (2023 entry), 36.1% to 39.6% (2024 entry), 36.4% to 41.6% (2025 entry). Children born September to November qualified slightly more often than those born June to August in each year, by 1.1, 0.3, 3.0 percentage points. That is a description of three cohorts, not evidence that birth month caused the difference.
What parents can take from it
A summer-born child's score is already adjusted, and in the published Bucks data the qualifying rate differed by at most 5.5 percentage points between birth months in any year. The CSSE allowance for 2027 entry will come from the September 2026 test, with results released after close of business on 12 October 2026.
What this does not show
- Panel A is arithmetic from CSSE's published formula and coefficients, holding raw marks equal. It is not a measured difference in how children performed.
- CSSE and Buckinghamshire use different tests, scales and standardisation methods; they are shown side by side, never combined. Buckinghamshire's standardisation method (GL Assessment) is not published.
- Birth-month shares are descriptive. Who sits the test, preparation and many other things differ between children; nothing here shows that birth month causes a result.
- Children outside the normal year group are left out of the Bucks birth-month counts (30 in 2023, 36 in 2024, 41 in 2025).
- CSSE's 2027-entry coefficients are not published yet. Other 11+ areas use their own rules and are not covered.
Cite this
ElevenPlusGenius (2026). 11+ birth month: how much the age adjustment adds, year by year. Published 27 September 2026, version 2026-09-27.1. https://elevenplusgenius.com/data/11-plus-birth-month
You are welcome to quote the findings and reproduce the chart with the credit above and a link to this page. Please cite the original publishers too; their figures remain theirs. Corrections and new releases are added as new versions, and earlier numbers stay listed in the change log below.
Method and sources
- Read CSSE's age coefficients for 2024, 2025, 2026 entry from its standardisation reports.
- Applied the published formula: points added = 1.5 × days younger × (English age coefficient + maths age coefficient), with days counted from 1 September to 31 August.
- Located the 2026-entry median among CSSE's 5,115 published standardised scores and found the percentile 13.96 points higher.
- Counted Buckinghamshire children scoring 121 or more in each birth month from the published score files, 2023, 2024, 2025 entry, ordering months September to August.
- Compared the three oldest months with the three youngest, pooling children within each group.
Sources
- CSSE: 2024 Entry Standardisation Report (October 2023)OfficialMean, standard deviation and age coefficient for each paper. Published 2023-10. Checked 26 September 2026.
- CSSE: 2025 Entry Standardisation ReportOfficialMean, standard deviation and age coefficient for each paper. Published 2024-10. Checked 26 September 2026.
- CSSE: 2026 Entry Standardisation ReportOfficialMean, standard deviation and age coefficient for each paper. Published 2025-10. Checked 26 September 2026.
- CSSE: Standardised scores from 2019 entry to 2024 entryOfficial2024 sheet. Published 2025-10. Checked 26 September 2026.
- CSSE: Standardised scores for 2025 entryOfficialevery candidate row. Published 2025-04. Checked 26 September 2026.
- CSSE: Standardised scores for 2026 entryOfficialevery candidate row; the file's own header says 2025Entry, but its name and the September 2025 test identify 2026 entry. Published 2026-04. Checked 26 September 2026.
- CSSE: Historical guidance sheet for 2026 entry (scores of last selective place offered, 1 March 2025)OfficialFootnote: no offer is made below a total standardised score of 303. Published 2025-10. Checked 26 September 2026.
- CSSE: Historical guidance sheet for 2025 entryOfficialFootnote: no offer is made below a total standardised score of 303. Published 2024-10. Checked 26 September 2026.
- CSSE: CSSE information guide for 2027 entryOfficialPage 5: key dates. Published 2026-03. Checked 26 September 2026.
- CSSE: Total number of CSSE 11+ examination candidates, 2010 to 2025 entryOfficialPublished 2025-04. Checked 26 September 2026.
- The Buckinghamshire Grammar Schools (statistics prepared by Buckinghamshire Council): Secondary Transfer Test scores by birth month, 2023 entryOfficialevery pupil score, by birth month. Published 2023. Checked 26 September 2026.
- The Buckinghamshire Grammar Schools (statistics prepared by Buckinghamshire Council): Secondary Transfer Test scores by birth month, 2024 entryOfficialevery pupil score, by birth month. Published 2024. Checked 26 September 2026.
- The Buckinghamshire Grammar Schools (statistics prepared by Buckinghamshire Council): Secondary Transfer Test scores by birth month, 2025 entryOfficialevery pupil score, by birth month. Published 2025. Checked 26 September 2026.
- The Buckinghamshire Grammar Schools (statistics prepared by Buckinghamshire Council): Qualifiers by location, 2023 entry (final)OfficialTotal and Buckinghamshire rows. Checked 26 September 2026.
- The Buckinghamshire Grammar Schools (statistics prepared by Buckinghamshire Council): Qualifiers by location, 2024 entry (final)OfficialTotal and Buckinghamshire rows. Checked 26 September 2026.
- The Buckinghamshire Grammar Schools (statistics prepared by Buckinghamshire Council): Qualifiers by location, 2025 entry (final)OfficialTotal and Buckinghamshire rows. Checked 26 September 2026.
- The Buckinghamshire Grammar Schools (statistics prepared by Buckinghamshire Council): Qualifiers by location, 2026 entry (spring)OfficialTotal and Buckinghamshire rows. Checked 26 September 2026.
Data and versions
- CSSE standardisation formula and published scores, 2024 to 2026 entry: version 2026-09-26.1, checked 26 September 2026 (historical). First transcription from the 11+ research data, 26 September 2026.
- Buckinghamshire Secondary Transfer Test scores and qualifiers, 2023 to 2026 entry: version 2026-09-26.1, checked 26 September 2026 (historical). First transcription from the 11+ research data, 26 September 2026.
- This story: version 2026-09-27.1, first published 27 September 2026.
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