National Security Letters (NSLs) are similar to administrative subpoenas and can be issued directly by elements of the executive branch without requiring prior approval from a court or grand jury. Importantly, NSLs authorize the imposition of nondisclosure orders (aka "gag orders") on the receiving party. Controversy about potential abuses of this authority has driven a range of legal and policy discussions. To address these concerns, both the public sector and the private sector have sought to document the usage of NSLs in aggregated form. However, each data source is limited in scope, time, and kind. In this paper, we consolidate the available data around NSLs and answer two questions: (1) what can the public effectively learn from the reported data and does this information suffice to assess the NSL usage? (2) how accessible is this data collection? We show that longitudinal trends in the usage of NSLs can be observed. For instance, we find a significant increase in NSL requests for non-US persons and that the policy reforms to decrease the mandated nondisclosure period appear to be effective. The observed trends suggest that the current transparency mechanisms are viable safeguards against the excessive use of NSLs. However, aggregating and normalizing the data requires manual reviewing, parsing, and validating. We even find inconsistencies within and across official data sources. Overall, the laborious data collection process hinders external and internal auditing efforts and demonstrates the need for a unified and more usable dataset for NSLs.

翻译：国家安全函（NSLs）类似于行政传票，可由行政部门直接签发，无需事先获得法院或大陪审团的批准。重要的是，NSLs授权对接收方施加保密令（即"禁言令"）。关于该权力可能被滥用的争议引发了一系列法律和政策讨论。为应对这些关切，公共部门和私营部门都试图以汇总形式记录NSLs的使用情况。然而，每个数据源在范围、时间和类型上都存在局限性。本文整合了现有关于NSLs的数据，并回答两个问题：（1）公众能从报告数据中有效获取哪些信息？这些信息是否足以评估NSLs的使用情况？（2）这些数据集合的可获取性如何？我们证明可以观察到NSLs使用情况的纵向趋势。例如，我们发现针对非美国公民的NSLs请求显著增加，而旨在缩短强制保密期的政策改革似乎行之有效。观察到的趋势表明，当前的透明度机制是防范过度使用NSLs的可行保障。然而，数据聚合与标准化需要人工审查、解析和验证。我们甚至在官方数据源内部及相互之间发现不一致之处。总体而言，繁琐的数据收集过程阻碍了内外审计工作，表明亟需为NSLs建立一个统一且更易用的数据集。