Exchange-traded funds have experienced tremendous growth over the past decade but not all of that growth is positive, according to new research from Vanguard.

As of March 31, more than $1.2 trillion was invested in about 1,400 U.S.-listed ETFs, according to the Valley Forge, P.A.-based fund firm, citing data from Strategic Insight’s Simfund. The report also revealed that U.S.-listed ETFs currently track more than 1,000 different indexes and, increasingly, ETFs have been launched using new indexes based on “narrow market segments and alternative weighting methods that often lack ‘live’ performance history.”

“Among the indexes being created for use in ETFs, more than half include back-filled or “back-tested” data from before the date the indexes were first publicly available, and it is often difficult for investors to discern which data are hypothetical and which are live,” according to the report’s authors Joel M. Dickson, Sachin Padmawar and Sarah Hammer.

“Perhaps not surprisingly, ETFs that track new indexes with back-filled data gather more assets in the first six months after ETF inception. Overall, it appears that index creation activity has been transformed from an exercise of providing investable benchmarks for different asset-class segments to one of providing ETFs a way to market and promote new products with ready-made indexes that might jump-start the acceptance and viability of new offerings.”