Predicting foreign-exchange markets has always been like piecing together a “mosaic” of different data sources — that’s just become even harder in recent weeks. Photo by Richard Drew/AP Photo/Postmedia files

The longest United States government shutdown on record is consigning currency traders to their worst year in decades as a dearth of economic data clouds the outlook for the dollar .

Foreign-exchange investors are on course for the poorest annual performance since 2005, according to a BarclayHedge index. The pinch was already being felt on Wall Street before the data vacuum, with Goldman Sachs Group Inc., Morgan Stanley and Bank of New York Mellon Corp. among those reporting a drop in currency trading revenues last quarter.

Amid the federal shutdown, crucial economic and

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