Index providers are resisting calls for more transparency on the methodology of their smart beta products, creating what some experts believe is a significant hurdle to investors’ ability to understand the risks.

Investment research firm Morningstar has identified 180 smart beta ETFs in Europe, with total assets under management of nearly €30 billion at the end of April. The European smart beta ETF market has been growing rapidly, and has more than doubled from €2.1 billion to €4.6 billion between January and September 2014, according to Invesco PowerShares. The firm also expects investors to double their allocations to these indices to 18% of assets by 2017.