It's an important tool for assessing trends in disease, behavior and public safety, both currently and for historians. But NVSS gets its data from other government agencies, which in some cases get them from others. Perhaps it shouldn't be surprising, but when it comes to tracking deaths caused by government — in this case, people killed by law enforcement officers — government agencies are doing a pretty lousy job. The Guardian reports:

Over half of all police killings in 2015 were wrongly classified as not having been the result of interactions with officers, a new Harvard study based on Guardian data has found.

The finding is just the latest to show government databases seriously undercounting the number of people killed by police.