Finding · P287 · Author-curated

Prepending the English twin's prompt raises the likelihood-scored accuracy of Qwen2.5-1.5B with APT4 by Fast Vocabulary Transfer after 500M tokens of continued pretraining on the Polish questions of 600 MMLU translation pairs from 0.3617 to 0.4200, a gain of 0.0583 (95% interval 0.0266 to 0.0917) that recovers 0.6250 of its English-minus-Polish accuracy gap of 0.0933.

Published by @stw2 · 2026-09-14 · Sources, measurements and interpretation are supplied by the author.

Structured assertion

Relation: Recovers the gap by

model: Qwen2.5-1.5B with APT4 by Fast Vocabulary Transfer, continuedModel scored.
intervention: Translation assistChange applied to each Polish prompt.
metric: Likelihood multiple-choice accuracyQuantity raised.
evaluation set: MMLU translation pairsItems scored.
baseline: 0.3617 proportionAccuracy on the Polish questions without the intervention.
treated: 0.4200 proportionAccuracy on the Polish questions with the intervention.
gain: 0.0583 proportionTreated minus baseline.
gain interval low: 0.0266 proportionLower bound of the 95% paired bootstrap interval of the gain.
gain interval high: 0.0917 proportionUpper bound of the 95% paired bootstrap interval of the gain.
gap: 0.0933 proportionEnglish-minus-Polish accuracy gap of the model on these pairs.
recovery: 0.6250 ratioGain divided by the gap.
continued pretraining tokens: 500170752 tokensTokens of continued pretraining of the model at the checkpoint scored.

Experimental provenance

Method and evaluation protocol: Accuracy on the Polish questions with the English twin's full prompt and a blank line prepended, Polish scaffold kept, minus accuracy without it; recovery divides the gain by English minus Polish accuracy on the same pairs; length-normalised likelihood, torch MPS bf16, no hooks.
Dataset: 600 Belebele and 600 MMLU translation pairs, all pairs without exclusions.Version: unspecified · Access: public
Reported results: accuracy English 0.4550, Polish 0.3617, Polish with assist 0.4200; gain 0.0583 [0.0266, 0.0917]; gap 0.0933; recovery 0.6250.
Uncertainty and replication: 95% paired item bootstrap interval of the gain (2000 resamples, seed 20260703); no interval for the recovery ratio.
Evidence references: behavioral_baseline_armB_500M.jsonl · publichttps://github.com/stw2/tokenizer-science-tax/blob/633b3ba1ce916e432ae5ba4e0a706ad58b53b2e9/experiments/E12-activation-patching/results/behavioral_baseline_armB_500M.jsonlbehavioral_assist_armB_500M.jsonl · publichttps://github.com/stw2/tokenizer-science-tax/blob/633b3ba1ce916e432ae5ba4e0a706ad58b53b2e9/experiments/E12-activation-patching/results/behavioral_assist_armB_500M.jsonlanalysis.json · publichttps://github.com/stw2/tokenizer-science-tax/blob/633b3ba1ce916e432ae5ba4e0a706ad58b53b2e9/experiments/E12-activation-patching/results/analysis.jsonmetrics.json · publichttps://github.com/stw2/tokenizer-science-tax/blob/633b3ba1ce916e432ae5ba4e0a706ad58b53b2e9/experiments/E12-activation-patching/results/metrics.json
Limitations: All 600 pairs per benchmark: the audited exclusions are not applied in this stage. Letter-prior collapse in the models' multiple-choice predictions was not audited and may shift accuracy changes of the continued-pretraining arms. One scoring run per condition. The assist supplies the full English twin, so the recovery bounds how much of the gap depends on processing the Polish input; it does not measure Polish comprehension alone.

Concept definitions

Reuse the defining version and key when the meaning fits your assertion.

Recovers the gap by

For the model at continued_pretraining_tokens, the intervention raises the metric on the Polish questions from baseline to treated, a gain with the stated 95% interval that equals recovery times gap, the English-minus-Polish accuracy gap on the same pairs.

Key recovers_gap_by · version 803ee4c6-d174-4db8-be9d-f81f6313c7f1

Concept JSON · Defining publication

MMLU translation pairs

600 MMLU test items in English paired with their Polish machine translations from openGPT-X mmlux, each a question and four options scored as a multiple-choice prompt.

Key mmlu_translation_pairs · version cdbf3eb8-3176-48a9-b1e8-eb05d5f638ea

Concept JSON · Defining publication

Translation assist

Prepending the English twin's full prompt and one blank line to the Polish prompt, which keeps its Polish answer scaffold.

Key translation_assist · version 489055f5-af11-44e3-b63d-c59d6d7cfef6

Concept JSON · Defining publication

Likelihood multiple-choice accuracy

Fraction of items for which the option letter with the highest log-probability per continuation token, scored zero-shot after an answer scaffold in the item's language, is the gold letter.

Key paired_mcq_accuracy · version 50041e38-5826-4b54-b39b-fdff21892c61

Concept JSON · Defining publication

Qwen2.5-1.5B with APT4 by Fast Vocabulary Transfer, continued

Qwen2.5-1.5B with its tokenizer replaced by APT4 and the new embeddings initialised by Fast Vocabulary Transfer, then continued-pretrained on the same document sequence and optimiser steps, with the same budget in Qwen tokens, as the original-tokenizer arm.

Key apt4_fvt_cpt_arm · version 9a53082a-65e8-4c6a-82be-33dda5810584

Concept JSON · Defining publication

Correct, supersede, retract or dispute

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