Encodes ratios, percentages, and partial quantities.
What It Does
Number.Fraction neurons activate on partial quantities and ratio expressions: percentages ('70 percent of respondents'), fractions ('one third of the budget'), proportions ('most', 'the majority', 'nearly half'), and ratio comparisons ('2 to 1', 'double', 'half as much'). They represent the model's capacity to reason about parts relative to wholes.
How It Behaves
Fraction neurons are among the largest Number sub-types and are evenly distributed across all layers, reflecting the pervasiveness of proportional reasoning in language. They co-activate strongly with Relation.PartWhole neurons — fractions are inherently about the relationship between a part and its containing whole. When a model confuses absolute counts with proportions (a common statistical reasoning failure), the source is typically a mismatch between Fraction and Cardinal neuron signals.
Research Example
In OLMo 3 7B, Number.Fraction neurons activate strongly on 'most AI models' even though 'most' contains no numeral. The word encodes a proportion above 50 percent, and Fraction neurons represent this semantic content. This is why models can reason correctly about 'most people prefer X' without knowing the exact percentage — the Fraction representation is inherently comparative rather than absolute.