We consider the setting where a user with sensitive features wishes to obtain a recommendation from a server in a differentially private fashion. We propose a "multi-selection" architecture where the server can send back multiple recommendations and the user chooses one from these that matches best with their private features. When the user feature is one-dimensional - on an infinite line - and the accuracy measure is defined w.r.t some increasing function 𝔥(.) of the distance on the line, we precisely characterize the optimal mechanism that satisfies differential privacy. The specification of the optimal mechanism includes both the distribution of the noise that the user adds to its private value, and the algorithm used by the server to determine the set of results to send back as a response. We show that Laplace is an optimal noise distribution in this setting. Furthermore, we show that this optimal mechanism results in an error that is inversely proportional to the number of results returned when the function 𝔥(.) is the identity function.
@InProceedings{goel_et_al:LIPIcs.FORC.2025.8, author = {Goel, Ashish and Jiang, Zhihao and Korolova, Aleksandra and Munagala, Kamesh and Sarmasarkar, Sahasrajit}, title = {{Differential Privacy Under Multiple Selections}}, booktitle = {6th Symposium on Foundations of Responsible Computing (FORC 2025)}, pages = {8:1--8:25}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-367-6}, ISSN = {1868-8969}, year = {2025}, volume = {329}, editor = {Bun, Mark}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FORC.2025.8}, URN = {urn:nbn:de:0030-drops-231353}, doi = {10.4230/LIPIcs.FORC.2025.8}, annote = {Keywords: Differential Privacy, Mechanism Design and Multi-Selection} }
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