I am trying to measure exchange rate volatility in some countries and I am using their currencies against euro. Problem is that one of them is Slovakia which has changed the currency in 2009 from crown to euro. From that year there is no way for me to measure its exchange rate against euro, because it is using euro (obviously). How would you suggest to solve this issue in the most correct way?
Approach I think of is comparing Slovakia currency for the whole time period against USD, but I am afraid this might introduce another problems (inaccuracies?) into my results. Also, if I do so, should I measure the other countries against USD and not EUR as well?