It's a meta study. That means the authors themselves haven't gathered any data but used data from other studies and done some number crunching on it. There have

It's a meta study. That means the authors themselves haven't gathered any data but used data from other studies and done some number crunching on it. There have already been hundreds of studies which have found links between loneliness factors and mortality. This study's authors contribution to the science is estimating the size, or strength, of that link.