https://www.nber.org/system/files/working_papers/w29444/w29444.pdf
Authors. Baldwin, R. & Freeman, R
Venue. NBER Working Paper 29444
How the Institute flags it
established 3 times frontier 2 times
This source carries more than one flag across the corpus. That is not an inconsistency to resolve: the same work can be established evidence for one claim and frontier evidence for another, and the flag belongs to the use rather than to the source.