Surprisal reveals diversity gaps in image captioning and different scorers change the story

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Hauptverfasser: Ilinykh, Nikolai, Dobnik, Simon
Format: Preprint
Veröffentlicht: 2025
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author Ilinykh, Nikolai
Dobnik, Simon
author_facet Ilinykh, Nikolai
Dobnik, Simon
contents We quantify linguistic diversity in image captioning with surprisal variance - the spread of token-level negative log-probabilities within a caption set. On the MSCOCO test set, we compare five state-of-the-art vision-and-language LLMs, decoded with greedy and nucleus sampling, to human captions. Measured with a caption-trained n-gram LM, humans display roughly twice the surprisal variance of models, but rescoring the same captions with a general-language model reverses the pattern. Our analysis introduces the surprisal-based diversity metric for image captioning. We show that relying on a single scorer can completely invert conclusions, thus, robust diversity evaluation must report surprisal under several scorers.
format Preprint
id arxiv_https___arxiv_org_abs_2511_04754
institution arXiv
publishDate 2025
record_format arxiv
spellingShingle Surprisal reveals diversity gaps in image captioning and different scorers change the story
Ilinykh, Nikolai
Dobnik, Simon
Computation and Language
We quantify linguistic diversity in image captioning with surprisal variance - the spread of token-level negative log-probabilities within a caption set. On the MSCOCO test set, we compare five state-of-the-art vision-and-language LLMs, decoded with greedy and nucleus sampling, to human captions. Measured with a caption-trained n-gram LM, humans display roughly twice the surprisal variance of models, but rescoring the same captions with a general-language model reverses the pattern. Our analysis introduces the surprisal-based diversity metric for image captioning. We show that relying on a single scorer can completely invert conclusions, thus, robust diversity evaluation must report surprisal under several scorers.
title Surprisal reveals diversity gaps in image captioning and different scorers change the story
topic Computation and Language
url https://arxiv.org/abs/2511.04754