An author’s fingerprint isn’t in their rare words — it’s in how often they use the tiny ones: ‘the’, ‘and’, ‘of’, ‘that’. Those rates are unconscious and hard to fake. Burrows’s Delta measures the distance between a mystery text’s little-word profile and each candidate’s. Slide the mystery sample and watch it snap to its true author.
Burrows’s Delta (2002): take the most common FUNCTION words, express each author’s usage rate as a z-score (standardised across the candidates), and the Delta between two texts is the mean absolute difference of those z-scores. The mystery text is attributed to the candidate with the SMALLEST Delta. The instrument runs this over two authors with distinct function-word habits. A fail-loud self-check throws unless a clearly A-leaning sample gets a smaller Delta to A than to B — attribution by the unconscious tics, not the subject matter.
A tiny two-author demo on a handful of function words; real stylometry uses 100–500 words, many texts, and cross-validation, and can be fooled by deliberate imitation or genre. The z-score standardisation and nearest-Delta rule are exact (Burrows 2002).