Imagine I want to reconstruct a counterfactual euro-dollar exchange rate in 2010 using a synthetic control variable to assess the impact of some policy. Could I use, for example, the exchange rates from 2000 to 2009 as a sample to create my SC?
More generally, when it's impossible to use similar subjects (e.g. neighboring countries) for a control group, can the historical series of the variable of interests serve as a control group?