When is it preferable to use a currency hedged ETF over a none currency hedged ETF? There has been studies which have shown over the longer term currency hedging does not make a difference. 

> "However, we believe that studies and our own experience have
> generally shown that over long measurement periods, the returns of
> hedged portfolios have been similar to the returns of portfolios that
> have not been hedged." 
 - [TWEEDY, BROWNE FUND INC][1]

However there is clear evidence this is not the case over the short term. Take Japan for example, the two largest ETFs `EWJ` in red and the currency hedged `DXJ` in blue. [![EWJ vs DXJ total return][2]][2]. There are clear periods where one variant is more a preferable investment over the other.

Using interest rates, currency rates and some other metrics how can you decide to use a currency hedged ETF or not?


  [1]: http://www.tweedy.com/resources/library_docs/papers/HowHedgingOct2014Fund.pdf
  [2]: https://i.sstatic.net/1hj7U.png