Pollack, Adam B.Sue Wing, IanNolte, Christoph2024-02-212024-02-212022-12A.B. Pollack, I. Sue Wing, C. Nolte. 2022. "Aggregation bias and its drivers in large‐scale flood loss estimation: A Massachusetts case study" Journal of Flood Risk Management, Volume 15, Issue 4. https://doi.org/10.1111/jfr3.128511753-318Xhttps://hdl.handle.net/2144/48129Large‐scale estimations of flood losses are often based on spatially aggregated inputs. This makes risk assessments vulnerable to aggregation bias, a well‐studied, sometimes substantial outcome in analyses that model fine‐grained spatial phenomena at coarse spatial units. To evaluate this potential in the context of large‐scale flood risk assessments, we use data from a high‐resolution flood hazard model and structure inventory for over 1.3 million properties in Massachusetts and examine how prominent data aggregation approaches affect the magnitude and spatial distribution of flood loss estimates. All considered aggregation approaches rely on aggregate structure inventories but differ in whether flood hazard is also aggregated. We find that aggregating only structure inventories slightly underestimates overall losses (−10% bias), and when flood hazard data is spatially aggregated to even relatively small spatial units (census block), statewide aggregation bias can reach +366%. All aggregation‐based procedures fail to capture the spatial covariation of inputs distributions in the upper tails that disproportionately generate total expected losses. Our findings are robust to several key assumptions, add important context to published risk assessments and highlight opportunities to improve flood loss estimation uncertainty quantification.enThis is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. © 2022 The Authors. Journal of Flood Risk Management published by Chartered Institution of Water and Environmental Management and John Wiley & Sons Ltd.https://creativecommons.org/licenses/by-nc/4.0/Aggregation biasFlood loss uncertaintyFlood risk estimationFlood risk managementFlood risk mappingOther agricultural and veterinary sciencesHydrologyPhysical geography and environmental geoscienceCivil engineeringArticle2024-01-1010.1111/jfr3.128510000-0001-6642-0591 (Pollack, Adam B)0000-0001-7827-689X (Nolte, Christoph)765181