Let's think about what this implies for all historical data about global stocks. In order to combine data in different currencies, it's necessary to convert currencies. For major currencies in normal times, when the nation is participating in a big way in international trade and currencies exchange rates are determined by market forces, this is probably straightforward. But it is not all that rare to have situations like Venezuela, where the government decrees and artificial, fictitious exchange rate in which (usually) the value of its currency is decreed to be much more than it really is.
It's not completely fictitious, because the official exchange rate is what the government uses in its own cross-border transactions. In the case of Venezuela the situation is beyond my ability to understand or track, but there are perhaps four exchange rates, and a quick Google search, probably outdated already, says that "The Venezuelan bolivar currently trades around 710 per U.S. dollar under the Dicom exchange rate and at 10 under the Dipro rate, Venezuela's other official rate. On the black market, however, a dollar can fetch around 3,000 bolivars." I imagine that if you sell the Venezuela government goods priced at $1, they probably pay you 10 bolivars and not 3,000.
So when people compile international financial data, what do they use as the exchange rate? My guess is that the easily obtained data are the official exchange rates, and there probably is no easy or authoritative reference for the black market rate if there is a black market.
What's interesting to me is that in this particular case, it really makes a difference. I'd have guessed, incorrectly, that it couldn't matter much. While I imagine the essential features of past datasets for international stock prices are OK, it does make you wonder how many Venezuela-like situations might be included in that past data. But perhaps Dimson and Marsh, who compiled "Triumph of the Optimists" and the Credit Suisse Global Return yearbooks, allow for this.
Annual income twenty pounds, annual expenditure nineteen nineteen and six, result happiness; Annual income twenty pounds, annual expenditure twenty pounds ought and six, result misery.