Whether they can be called smart beta or not, currency-hedged exchange-traded funds are attracting the attention of investors and have doubled in number against a backdrop of a fluctuating US dollar in the past year.

Since July 2014, the dollar has been strengthening against many currencies, including the euro and the yen. At the beginning of this month, the euro was trading below $1.10, a level not seen since 2003. The correlation between the relative strengths of the three currencies and central banks' quantitative easing initiatives is expected: the dollar's rise has coincided with the US Federal Reserve ending quantitative easing while the European Central Bank is starting a sovereign bond-buying programme and the Bank of Japan is increasing its QE efforts.