{"about":{"site":"https://codewithpapers.app","non_affiliation":"Code with Papers and Syntology are not affiliated with, endorsed by, or sponsored by Papers with Code, Meta, or the pwc-archive mirror.","licence":"CC BY-SA 4.0","licence_url":"https://creativecommons.org/licenses/by-sa/4.0/legalcode","attribution":"https://codewithpapers.app/attribution","modified":"archive material modified by Syntology; see the attribution page"},"url":"/paper/the-fractions-skill-score-doesn-t-always","title":"How to derive skill from the Fractions Skill Score","arxiv_id":"2311.11985","date":"2023-11-20","proceeding":null,"authors":["Bobby Antonio","Laurence Aitchison"],"abstract":"The Fractions Skill Score (FSS) is a widely used metric for assessing forecast skill, with applications ranging from precipitation to volcanic ash forecasts. By evaluating the fraction of grid squares exceeding a threshold in a neighbourhood, the intuition is that it can avoid the pitfalls of pixel-wise comparisons and identify length scales at which a forecast has skill. The FSS is typically interpreted relative to a `useful' criterion, where a forecast is considered skillful if its score exceeds a simple reference score. However, the typical reference score used is problematic, as it is not derived in a way that provides obvious meaning or that scales with neighbourhood size, and forecasts that do not exceed it can have considerable skill. We therefore provide a new method to determine forecast skill from the FSS, by deriving an expression for the FSS achieved by a random forecast, which provides a more robust and meaningful reference score to compare with. Through illustrative examples we show that this new method considerably changes the length scales at which a forecast would be regarded as skillful, and reveals subtleties in how the FSS should be interpreted.","url_abs":"https://arxiv.org/abs/2311.11985v3","url_pdf":"https://arxiv.org/pdf/2311.11985v3.pdf","source":{"archive":"pwc-archive (Hugging Face), CC BY-SA 4.0","snapshot":"2025-07-28","licence_url":"https://creativecommons.org/licenses/by-sa/4.0/legalcode","row_kind":"links_only","authors_date_abstract":"arXiv metadata, CC0 1.0 (https://info.arxiv.org/help/license), from the Kaggle arXiv metadata snapshot of 2026-09-12"},"code_links":[{"paper_slug":"the-fractions-skill-score-doesn-t-always","repo_url":"https://github.com/bobbyantonio/fractions_skill_score","is_official":1,"mentioned_in_paper":0,"mentioned_in_github":0,"framework":"none","reach":null}],"tasks":[],"methods":[],"datasets_introduced":[],"methods_introduced":[],"results":[],"syntology":{"syntology_url":null,"atlas_url":null,"mcp":null,"developers":"https://syntology.ai/developers"},"arxiv_metadata":null,"syntology_extracted_results":null}