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The enterprise Wi-Fi networks enable the collection of large-scale users' mobility information at an indoor level. The collected trajectory data is very valuable for both research and commercial purposes, but the use of the trajectory data also raises serious privacy concerns. A large body of work tries to achieve k-anonymity (hiding each user in an anonymity set no smaller than k) as the first step to solve the privacy problem. Yet it has been qualitatively recognized that k-anonymity is still risky when the diversity of the sensitive information in the k-anonymity set is low. There, however, still lacks a study that provides a quantitative understanding of that risk in the trajectory dataset. In this work, we present a large-scale measurement based analysis of the low-diversity risk over four weeks of trajectory data collected from Tsinghua, a campus that covers an area of 4 km ² , on which 2,670 access points are deployed in 111 …