Most of the investments in ETPs are concentrated among the top four providers: BlackRockiShares, Vanguard, State Street Global Advisors (SPDR) and Investco PowerShares.
About 1, 200 ETFs are on the U.S. market, with BlackRock's iShares recently commanding a market leading share of 44 percent of all ETF assets, according to the research firm ETFGI.