The data used was described in detail here
papers.ssrn.com/sol3/papers.cf… as well as in MGLC (Univ of Chicago Press).
As to my research being discredited,
crimeresearch.org/2014/11/do-rig…
crimeresearch.org/2018/06/more-m…
If you think that the @nytimes is missing mass public shootings,
1) Identify the cases
2) show that they don't fit the definition.