The Social Security Death Master File, an index of 90 million deaths from survivors, hospitals, funeral homes and state offices that includes names, Social Security numbers and dates of death, was not made public until a legal ruling in 1980, The New York Times reported.
After the legal ruling, the list was updated weekly and although it is not comprehensive nor 100 percent accurate, researchers used the data. Ten years ago, information from states was also included.
Last year, the federal agency responded to complaints concerning identity theft and removed more than 4 million names off the list from the states, The Times said.
However, confirming deaths from each state would be costly and time consuming, and epidemiologists said the only alternative death index was from the Centers for Disease Control and Prevention, but their index is typically more than one year old.
For example, Jesse D. Schold of the Cleveland Clinic in Ohio said the omissions had already compromised his investigation into mortality rates among living kidney donors.