  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.
