Before reaching any conclusions I might ask: Looks like the data set is only NYSE, AMEX, NASDAQ... A. have those exchanges strengthened their listing criteria

Before reaching any conclusions I might ask: Looks like the data set is only NYSE, AMEX, NASDAQ... A. have those exchanges strengthened their listing criteria, making it harder to list B. Has the number of firms listed on alternative exchanges taken up the slack? C. Are companies deciding to list in other jurisdictions because of inversions or regulatory concerns D. Looks like they exclude certain types of securities - what are the securities excluded? ADRs, MLP, REITs, etc could also be part of the story E. What about the semi-private 144a market? How does one factor that in? F. How has the elmination of regional stock exchanges played a role?