The Political Polarization Index (The US)

The graph is from this paper by Marina Azzimonti, she explains

I follow a similar methodology to that in Baker, Bloom, and Davis (2013) in constructing the po- litical polarization index (PPI). In particular, I use a search-based approach that measures the fre- quency of newspaper coverage of articles reporting political disagreement about government policy. The identification assumption underlying the index is that greater media-coverage of ideologically divisive issues or legislative gridlocks is associated with higher polarization episodes (p. 3).